Either The King of Fighters 2002, or Garou/Fatal Fury: Mark of the Wolves. I would have no problem paying $40 for portable versions with online play.
Man, I wish SNK made 3DS games. :(
This is completely off-topic, but dude, get yourself a Neo Geo X. Got one a couple months back and it's absolutely amazing. Although I have to admit I don't ever use the handheld (it's just way too awesome to play those games on a big screen).
It would probably have to be a new game considering you can't pop in CDs. using Wifi ports would work... I wouldn't be crazy and say P4 Golden or Muramasa simple because the 3DS cannot power those games. Realistically the 3DS CAN render PSP and N64 though. I'd like PSP games that don't make it stateside because of poor PSP sales here; eg: Valkyria Chronicles 2,3; Akiba's trip; Yakuza; Persona 3 FES; Idolm@ster; Clannad
Also N64 games that the 3DS obviously has enough juice to power but somehow don't get released LoZ: Majora's Mask WWF No Mercy (for obvious licensing issues) Doujin the Giant --- That's not suspicious at all! PSN: anthler316; 3DS: 3840-5352-3266