Best hp:exp ratios in the game?

Nope. Even though most of the time, weakest monsters have the best hp/exp ratio, the second form of Papulatus has the highest hp/exp ratio.

1.64 Papulatus (2nd form)
2.67 Snail

Credits to WhiteasIce here. (gotta scroll down a bit)

That's not a "proper" hp/exp ratio though, because
1) it doesn't include the 1st form, which you also have to kill, and
2) neither form is self spawned anyway.