Hmm, tough one... I would say Natsu since it seems fire doesn't affect him all too well.
I know this also applies to Portgas D. Ace but since he's a Logia composed of fire when afflicted on. I don't see any reason why Natsu wouldn't just consume him. lol
Plus Natsu gets even more powerful or rejuvenates his energy when consuming flame. So I see Ace at a major disadvantage.
Natsu is the main character from a BA manga called Fairy Tail. I think the guy that writes it is the same guy who create Rave Master. I could be wrong. IMO they should make this into an anime.
P.S. Glad to see Natsu in a versus thread. 馃槉