It's barely a fighting game, but Divekick is a great introduction to the genre (barring the super in-jokes). I've gotten a lot of people who otherwise wouldn't touch a fighting game to play Divekick for hours.
Beyond that, it's largely dependent on their history with the genre. If they were fans of Street Fighter II then I see no reason why you couldn't get them in to Third Strike. Obviously the mechanics have changed but give them Ryu and Ken and 90% of people will be happy. If they have little to no history, there are a couple of routes you can take based on their interests. If you're trying to hook people, you want to show them something that's either easy on the eyes (old sprite-based games aren't going to interest anyone if they have no nostalgia for them) or easy to pick up and immediately do cool, flashy stuff with.
Mortal Kombat isn't for everyone but the new one (from 2010) is fairly approachable and looks good. If they can tolerate anime, Blazblue: Chrono Phantasma is gorgeous and (while incredibly deep and mechanics-heavy at high levels) fun as a mashing game. If you're down for 3D, I'd say play the original Soul Calibur or 2 (both HD-ified on X360). Very mash-friendly but still rewarding if you dig a little deeper. I wouldn't touch anything Neo until they're comfortable with the basic mechanics and are looking for something a little more meaty.