The Artificer class from Unearthed Arcana gains a so-called Mechanical Servant at level 6. It adopts the stats of a beast of CR 2 or lower and makes a few changes (such as changing the type to construct or gaining darkvision). See the linked document for details.
The question arises: which beasts are a valid choice for this feature? An answer to this question could serve many future readers considering to play as an Artificer. Of course, the ultimate goal here would be getting the most out of this feature, mechanically speaking. However, doing so requires knowledge of what is even available for choice at all, which can be a daunting task if you want to consider all beasts in all rule books.
Here is the relevant passage from the UA document:
Select a Large beast with a challenge rating of 2 or less. The servant uses that beast’s game statistics, but it can look however you like, as long as its form is appropriate for its statistics. It has the following modifications: […]
Best Answer
I will list all valid beasts below, ordered by their appearance in each book and sorted by source.
Creatures with fly speed or swim speed (none of which are forbidden by the description in the Artificer class) will be marked accordingly. Creatures with other noteworthy traits (such as being unable to breathe outside of water or possessing an innate climb speed) will have a note attached.
Monster Manual
Volo's Guide To Monsters
Mordenkainen's Tome of Foes
There are no beasts in this book.
Tales from the Yawning Portal
Giant Crayfish CR2, p.235
Storm King's Thunder
Crag Cat, CR1, p.240 (spell turning, nondetection, lowish stats for CR 2, stealth expertise, special crawford tweet)
Other sources
Apparently, some of the Adventurer's League books feature their own, exclusive creatures. I don't have any of those books - anyone who does, feel free to add them to this list.
The following is a somewhat subjective rating. You might want to use a creature not listed below if you're looking for specific traits or features.
Aside from that, these are what I consider the strongest or most versatile creatures (top 5 are emphasized):