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)
1.64 Papulatus (2nd form)
2.67 Snail
Credits to WhiteasIce here. (gotta scroll down a bit)

