Belloq, clearly. He's the one really suited to being a recurring nemesis; the others are mostly closely tied to the specific adventures or locales in which Indy encounters them, and few really darkly mirror Indy so well as Belloq himself observes he does. Note, too, that Raiders itself indicates they've known one another for a while and crossed paths before; the adventures around the Chachapoyan Idol and the Ark of the Covenant are just two of presumably many times they've competed in the field. As we now know, an episode of The Young Indiana Jones Chronicles was even planned that would show the beginning of their "relationship" in the early 1920s.
Location: The sun is shining a little stronger. Time to hit the road and drift southbound.
I would like to see Mola Ram return.
He is the most evil of the villians, and his life ended so anti-climatic. We have Belloq's head exploding, Donovon withers away, and Spalko's brain fries, but poor Mola Ram just falls off a cliff and gets ate by crocs.
I voted Irina. I would like to see more of her fencing and so called "telepathic abilities". Plus, she is sexy. IMO, Belloq and Donovan are just boring, they are just rich people without any "special" abilities.