No brainer for me: Mola Ram.
This man is pure evil, dabbling in voodoo, leading a cult, enslaving children...oh and incinerating people after ripping out their hearts.
He certainly
looks imposing (and impressive), with his draped ceremonial robes and dramatic horned headdress, complete with shrunken head. I just prefer watching a more hands-on villain, one who isn't afraid to also get down and dirty and actually get involved in a scuffle with Indy...and what better place to have a fight in an Indiana Jones film than dangling off the side of a cliff?!
But Belloq comes in at a very close second, since he is evil in a much more understated, suave, debonaire kind of way - seductively evil, he almost lures one in and then uses them for his own gain, no matter the cost.
Plus his head explodes..so his death is pretty memorable too. Hah.