Drac gives about 1m total exp from MvP and regular exp combined according to RMS, not worth it for the 320k hp he has if you ask me. Plus his Familiar swarm and cloaking are annoying. xD
Phreeoni is pretty easy and I'm pretty sure he gives more exp than RMS says he does on here. You could try Maya for luls I guess, they both have pretty low HP.
Pharaoh does give nice exp but his spells could be nasty to get around.
Basically I'd say Atroce has the best Exp to HP ratio if you factor in how easy he can be compared to others.