What are the best transformations?

[AdamMeyers]

The form-changing spells (beast shape, undead anatomy, elemental body, etc.) give different abilities depending on the monster mimicked. With 4 official bestiaries and countless 3rd party ones, which monsters are the best to change into? I'm looking for any incarnation if the spells (I to IV) and for any spellcasting situation (Druids, full wizards looking for defense, Eldritch Knights looking for offense or utility, etc.)

Thanks in advance.

[Lemonfresh]

Druids like Dire Tiger and Allosaurus for Combat, Dire Badgers for Burrowing through stone, Giant Octopus for underwater combat and Air Elementals for flying.

[EsperMagic]

Monstrous Physique 2 Four armed gargoyle

[Unruly]

While looking into making a Cave Druid a while back, someone else popped into my thread with an idea that took advantage of being able to Wild Shape into an Ooze and get some massive damage out of it. You can find the build here. The guy called it the Conqueror Ooze, and it's based on multiclassing Druid/Monk.

If it actually works the way he was figuring it to work, it would have some crazy-high damage potential. But that's about it, and it requires pretty much a full character investment to pull off. Not sure if it counts as "best," but it's at least "interesting."

[Xein]

This isn't mine but I love it so much I have it in my favorites.