It's a hard question. Jafar as a Genie has endless power, he can do anything he wants - but also, when he hasn't got a master, he's just in the lamp. But when he's outside, he has a master who could stop him. (OK, if I was his master I would never stop him or say anything wrong to him, because I would to fear him, but they could be someone who would do it... But I don't want to imagine Jafar's mad in that case!) So altough his power is endless he has limitations. Even Hades is a god. The lord of the Underworld, the god of deaths. They're similar strong, but I would say afore Hades, but I can imagine Jafar's winning. And that's true, Hades can be so mad, too, but Jafar's a bit more... Scary, maybe? I don't know, maybe because he's more serious than the joke-king Hades. OK, than I say, Jafar would win...
OK, I really don't know. There's chance for both of them.
I guess! Hades would win. Jafar's a genie, he has endless power. True. But he has so much more rules than Hades, becuase a genie can't kill people and can't leave his bottle. So he without a wish to become free would never perfectley endless powered. But as a free genie his power would be not that strong like before. So Hades would win - and I didn't speak about that Jafar as a genie can die (if we scetch his bottle), but Hades as a God can't.
Hades, as a God, should be far more powerful than any genie.
Hades wins! Hades is a god, and even if he wasn't Jafar as a genie can't kill people. Of course, there are always ways around this, but Jafar still hasn't accomplished killing mere mortals so how is he going to find his way around 2 major obstacles: killing someone AND making a god mortal. Besides, I don't see how Jafar is going to inflict much pain on Hades when characters like Jasmine, Iago and even Abis Mal have been able to dodge Jafar's attacks. Not to mention, Hades can teleport!
Plus, Jafar has a major weakness: his lamp. To quote The Genie, "You destroy Jafar's lamp, you destroy Jafar". This shouldn't be a problem for Hades when he burns and melts everything he touches and getting his hands on it will be easy when again, he can teleport! And Jafar, being as cocky as he is wouldn't even take precaution just like he didn't in the finale of The Return of Jafar.
I think Hades would win, he is a god
Both are very powerful but they also have their own Achilles Heels. Jafar as an unfreed genie is more powerful with a lamp but if his lamp gets destroyed then he is screwed. Hades as a god could still be trapped in Tartarus or if Jafar is especially creative, make him drink the same elixir he fed to Hercules as a baby and that will take away his godhood.