Wild Shape/Summon Beast.

CR 0
At CR 0 you’re not looking for a combat form. CR 0 includes creatures which aren’t dangerous, or are only dangerous in great numbers (I.E. anything with Pack Tactics). Instead, look here for forms you want to use for scouting and infiltration.
 
Flight is typically your best bet for scouting, which makes the Owl the best option. Underwater, the Octopus is your best bet. If you need to climb instead of fly for some reason, Spider is a good choice. If you need to get around unnoticed in a city, Cat or Rat are both good options in case you’re spotted.
 
Cranium RatVGtM: Darkvision, Illumination, Telepathy, and Telepathic shroud. Unfortunately, with no Stealth bonus it may not work as a scout form.
DeerMM: Great speed, but if you just want to be fast you’re probably better served by something that can fly.
EagleMM: Fantastic fly speed, the best attack at this CR, but any Druid who can Wildshape into an Eagle can also Wildshape into a Giant Owl, which is considerably better. Unusable until level 8.
FrogMM: Swim speed and 30 ft. Darkvision. Crab is strictly better, so Frog is only a good choice where you need to look inconspicuous in the environment. Someone seeing a crab in a swamp would likely take notice. Unusable until level 4.
Giant Fire BeatleMM: Blindsight, and you glow. Magical light is easily available, and torches are cheap, so your party shouldn’t have issues with illumination. If you need to see in the dark, turn into something with better Blindsight or with Darkvision.
GoatMM: Passable damage and knockdown if you hit with Charge, but it’s hard to use charge more than once in the same combat.
HawkMM: Eagle is strictly better. Unusable until level 8.
LizardMM: Darkvision and a climb speed, but no stealth.
OwlMM: Good stealth, huge Darkvision range, Keen Hearing and Sight, and a fantastic fly speed. Unusable until level 8, but even at that level it remains an absolutely fantastic flying scout form.
RatMM: Darkvision and Keen Smell. 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. However, this advantage doesn’t apply to monsters, cats, or people who just don’t like rats.
RavenMM: Decent fly speed and decent Perception, and you might be able to speak using Mimicry, but other flying options are better for stealth and scouting. Unusable until level 8.
ScorpionMM: Slow, a tiny bit of blindsight, and a little bit of poison. Unfortunately with no stealth or Dexterity you’re very easy to spot.
SpiderMM: Good stealth, Darkvision, and better climbing than anything else thanks to Spider Climb. Flight is generally a better option, but sometimes you need to hang on a ceiling.
VultureMM: Keen Sight and Smell, and Pack Tactics on a flying creature, but Eagle is better for combat and Owl is better for scouting. Unsuable until level 8.
 
CR 1/8
This CR is a difficult middle-ground, mostly populated by larger version of CR 0 options. CR 0 has better scouting options because smaller creatures can more easily access small areas, and all Druids have access to higher CRs to get better combat forms.
 
Flying Snake is really the only useful option here because it can do so much with a single form.
 
Blood HawkMM: Flight, Keen Sight, and Pack Tactics, but still can’t compete with better combat forms available to Druids of the same level. Unusable until level 8.
Giant CrabMM: Blindsight, and better swim speed and better Stealth than the basic Crab, plus (obviously) better attacks. 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. Unusable until level 4.
Giant RatMM: No special movement, mediocre Stealth. Keen Smell, but plenty of better options have that. Oh, and Pack Tactics to back up its mediocre attack.
MastiffMM: Keen Hearing and Smell, and a bit with a knockdown effect. Plus, since it’s a dog, you can probably go unnoticed in civilized areas.
MuleMM: Camel is faster, but I guess sometimes you need to carry a ton of loot.
Poisonous SnakeMM: Unusable until level 4, and by then you have better options
PonyMM: Camel is faster.
StirgeMM: Flying Snake is strictly better. Attach seems tempting, but it makes you extremely vulnerable to being knocked out of Wild Shape.
 
CR 1/4
Everyone but Moon Druids will start with this CR as their best option from levels 2 and 3. 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. Against groups, consider Giant Owl, Wolf, or Velociraptor.
 
Axe BeakMM: Good speed and a decent attack, but no special abilities and absolutely awful AC.
BoarMM: Charge is tempting due to the knockdown effect, but the DC is very low. Poor AC, poor hit points, poor damage, and Relentless only works once per rest.
Constrictor SnakeMM: Swim speed, blindsight, and Constrict. Constrict is an absolutely fantastic way to take down single targets, especially if they have trouble passing the escape DC. Unfortunately it’s unusable until level 4, at which point Land Druids will be using Crocodile, and Moon Druids will be using the Giant Constrictor Snake.
CowVGtM: A decent charger, but with poor AC and low hit points it won’t survive moving away to make another charge.
Draft HorseMM: Decent speed and a very solid attack, but garbage AC.
Giant BatMM: A bizzarrely large number of hit points, but terrible damage output and at large size a very poor scout. Unusable until level 8, by which point everyone has better options.
Giant OwlMM: Decent damage and Flyby, which are great on a flying form. Unusable until level 8, at which point you have better options.
Giant Poisonous SnakeMM: Swim speed, reach, a bit of blindsight, and poison damage. Unfortunately the poison DC is very low, and it’s unusable until level 4, at which point you have better options.
Giant Wolf SpiderMM: Fantastic stealth, and all of the climbing ability you expect from a spider. Decent poison damage, but the DC is low.
Riding HorseMM: Decent damage attack, and obviously ridable, but everything else is bad.
WolfMM: Keen Hearing and Smell, and Pack Tactics to back up the Wolf’s attack. The damage isn’t great, but it applies a knockdown effect on every hit. Decent AC for an animal at this CR, but not a ton of hit points.