ROBOT uses the color coding scheme which has become common among Pathfinder build handbooks, which is simple to understand and easy to read at a glance. I also won't cover Unearthed Arcane content because it's not finalized, and I can't guarantee that it will be available to you in your games.
The advice offered below is based on the current State of the Character Optimization Meta as of when the article was last updated. Keep in mind that the state of the meta periodically changes as new source materials are released and this article will be updating accordingly as time allows.
Wild shape is an exciting and versatile Druid ability which a lot of people want to use, but which is easy to use badly. There is a lot of confusion around the rules of Milkshake and its interaction with your racial traits and class features.
I strongly recommend listening to this episode of the Dragon Talk podcast, in which Jeremy Crawford spends 45 minutes discussing the minutiae of Milkshake and its interaction with other stuff. If you need to get around unnoticed in a city, Cat or Rat are both good options in case you're spotted.
Badger MM : Keen Smell and a burrow speed can be occasionally useful. Bat MM : Tiny size, flight, blind sight, and Keen Hearing make this an absolutely fantastic form for scouting.
Cat MM : Tiny size, a decent Stealth bonus, Keen Smell, and a climb speed. Cranium Rat GTM : Dark vision, Illumination, Telepathy, and Telepathic shroud.
Dear MM : Great speed, but if you just want to be fast you're probably better served by something that can fly. Magical light is easily available, and torches are cheap, so your party shouldn't have issues with illumination.
Jackal MM : Good speed, Keen Hearing and Smell, and pack tactics. Owl MM : Good stealth, huge Dark vision range, Keen Hearing and Sight, and a fantastic fly speed.
Quipped MM : Swim speed and Dark vision, but the Octopus is strictly better. Rats have a major in-world advantage because they're a common sight in populated areas, so it's easy to escape notice even if you're spotted by humanoids.
Vulture MM : Keen Sight and Smell, and Pack Tactics on a flying creature, but Eagle is better for combat and Owl is better for scouting. Weasel MM : Good Perception and Stealth, plus Keen Hearing and Smell.
Blood Hawk MM : Flight, Keen Sight, and Pack Tactics, but still can't compete with better combat forms available to Druids of the same level. Camel MM : Decent speed, and the first form which is definitely rideable by your allies.
At medium size it doesn't really make sense as a Scout form because it can't fit into small spaces, so it doesn't really have an obvious role. Giant Rat MM : No special movement, mediocre Stealth.
Giant Weasel MM : Dark vision and Keen Hearing and Smell, and good Stealth, but no special movement types. Mastiff MM : Keen Hearing and Smell, and a bit with a knockdown effect.
Mule MM : Camel is faster, but I guess sometimes you need to carry a ton of loot. Attach seems tempting, but it makes you extremely vulnerable to being knocked out of Wild Shape.
Circle of the Moon Druids will skip this CR entirely because they have access to considerably better options. Against single enemies, go for Giant Frog if they're small or smaller, or Velociraptor if they're not.
Axe Beak MM : Good speed and a decent attack, but no special abilities and absolutely awful AC. Boar MM : Charge is tempting due to the knockdown effect, but the DC is very low.
Constrictor Snake MM : Swim speed, blind sight, and Constrict. Constrict is an absolutely fantastic way to take down single targets, especially if they have trouble passing the escape DC.
Cow GTM : A decent charger, but with poor AC and low hit points it won't survive moving away to make another charge. Draft Horse MM : Decent speed and a very solid attack, but garbage AC.
Giant Badger MM : A burrow speed, Keen Smell, and Multi attack. Giant Frog MM : The frog's Bite and Swallow abilities are considerably more lethal than the Constrictor Snake's Constrict ability, but they only work on Small or smaller creatures.
Giant Owl MM : Decent damage and Flyby, which are great on a flying form. Pounce adds a nice knockdown effect to the Panther's claws, but it doesn't have the damage output to compete with other combat forms at the same CR.
Riding Horse MM : Decent damage attack, and obviously rideable, but everything else is bad. Velociraptor GTM : Better damage than the Wolf and Pack Tactics, but no trip effect.
Decent AC for an animal at this CR, but not a ton of hit points. By level 4 you also have access to forms with swim speeds, so Land Druids can use great options like the Crocodile and Reef Shark.
Several excellent flying options exist at this CR, but Moon Druids have better options at CR 1, and other druids can't select forms with a fly speed. The bite does decent damage, and until the target escapes you get advantage on your attacks against it.
The Warhorse works similarly, but does better damage, has better speed, and knocks targets prone on a charge. Giant Seahorse MM : Less damage and worse AC than the Reef Shark.
The damage on the wasp's sting isn't great, but the poison is very potent if your target fails the low save DC to resist it. Warhorse MM : Excellent speed and damage, and the charge effect is a great way to help your allies, though you have no way to capitalize on your enemies being prone.
Fortunately, Land Druids get this CR at level 8 when flying forms open up, so the Giant Vulture is the perfect answer to these problems. Against single enemies, the Giant Toad can quickly remove anything of medium size or smaller.
For other fights, the Giant Vulture does great damage, and the Dire Wolf has a nice knockdown effect. The Giant Spider's web is a fun utility, especially against big enemies with low AC.
Unfortunately, the Brown Bear's AC is relatively low for the CR, and it doesn't have enough hit points to compensate, so it's very difficult to maintain the form if you're drawing attacks. If you insist on using Brown Bear (and many people do) be absolutely sure to put of Bark skin before you go into combat.
Deinonychus GTM : An excellent charger, the Deinonychus can outdo the Dire Wolf's damage on a multi attack, but can only trip if it moves around, and you really want to charge in and out to try to knock your targets prone with Pounce and get the bonus action bite. Personally I think the Dire Wolf is a better option since you don't need to risk opportunity attacks to trip things, but the Deinonychus is still a potentially good option and if you can make it work their Deinonychus's damage output will be better.
Keen Hearing and Smell, and good Perception and Stealth for whatever reason. Giant Eagle MM : Spectacular fly speed, Keen Sight, and multi attack with two decent attacks.
Unfortunately, the Giant Vulture does considerably more damage and has Pack Tactics to back up its attacks. Giant Hyena MM : Low AC, but a decent pile of hit points.
The Giant Hyena's bite does decent damage, and with Rampage you can chew through groups of weak enemies very quickly, especially at low levels. Giant Octopus MM : Underwater this is the only form available at this CR, but it's a great option for combat, and it's even decent at stealth.
The attack bonus is decent, and the target doesn't get a save to resist its effect. The target can then either waste an action trying to escape, make attacks at disadvantage to cut its way out, or attempt to fight while restrained.
This is especially true against single enemies where the action economy tips heavily in the party's favor. Giant Toad MM : A bigger version of the Giant Frog, the toad adds poison damage to its bite, and increases the size of creature which you can swallow to medium.
Swallowing an enemy is still a great way to eliminate important single targets. Thanks to Pack Tactics, the Giant Vulture can reliably hit with sits Multi attack and deal impressive damage.
Tiger MM : Nearly identical to the Lion, but with Dark vision and a tiny bit more damage. Flying forms don't open up until level 8, but you'll still be on CR 2 at that point, so if you need to fly, Giant Vulture is still your best bet until level 9, at which point the Quetzalcoatl us will easily outdo the Giant Vulture.
Alxasaurus MM : Giant Elk works in much the same way, and does considerably more damage. Aurochs GTM : Slightly better damage output than the Giant Elk, but much less durable.
Giant Boar MM : The Dire Wolf is arguably better since it can trip every time it attacks, and it has comparable defenses and damage. If you have an ally whose good at Shoving and doesn't mind spending one of their attacks to help you out, Hooves is a ridiculous pile of damage.
If you use hit-and-run tactics to keep Charge/Pounce in play, the Elk's superior movement speed is crucial for positioning, and its higher AC is better for opportunity attacks when you move away. CR 3 Ankylosaurs serves as a Defender, while Giant Scorpion is great for holding stuff in place and murdering it.
The sting does fantastic poison damage, even if the target passes their save. Even the scorpion's AC is decent, and with blind sight you can easily handle invisible or obscured enemies.
Killer Whale MM : Fantastic blind sight and decent damage, but not as useful in combat as the Giant Octopus. Stegosaurus is the best new option, but its raw damage output probably can't compete with the Giant Scorpion's ability to hold foes in place and wear them down.
Elephant MM : Worse AC than the Giant Scorpion, comparable damage, and only a few more hit points. Trampling Charge is decent, especially since you can get a free attack, but the DC is lower than the knockdown effect on the Dire Wolf's bite, so even that doesn't make the Elephant good.
Stegosaurus GTM : Better damage and AC than the Elephant, but no special abilities. Against multiple foes, consider the Brontosaurus for its amazing damage output and reach.
Giant Shark MM : Great damage, blind sight, and a huge pile of hit points. Mammoth MM : Very similar to a Giant Elk with more hit points.
All the Elementals in the Monster Manual are CR 5, making them the highest-CR options available to Druid until level 18. However, this option consumes both of your Wild Shapes uses, which can make it a risky choice if you're not planning to rest soon.
The damage is on par with other elementals, and Whirlwind is fun but not terribly powerful. A level of Barbarian or Monk to pick up Unarmored Defense goes a long way for Moon Druids because it applies while Wild Shaped, according to Mike Merely.
The Dragon born racial feat Dragon Hide gives you an ability score increase (override by Milkshake ; that part is clear), but it's not totally clear how the rest of the feat works. In my opinion, anything which is part of your character's body like claws, natural armor, or damage resistances doesn't work while in Milkshake.
Anything your character knows, like proficiency in something, specific training granted by a feat like Mobile or Actor, or something inherent to your character's personality, culture, or anything else non-physical like the Lucky feat or the halfling Lucky trait should still apply in Milkshake. Mike Merely isn't the official rules guy, so his opinions are roughly on par with AA well-versed player.