I like what nitro does to a hoppy or bitter beer. I've done an IPA on nitro once before (granted I had an imperial IPA on CO2 at the same time). Overall, I think IPAs are best in CO2, but nitro will soften 'em up. For the same reason, I like an APA on nitro every now and then. Progressing in the same direction, I like even more to have any variety of English bitter on nitro.
I think that temp plays a big role with nitro beers. If you can serve them in the 50's rather than the 30's/40's, I think it helps. With a hoppy beer, where the lack of carbonation cuts down the crisp, drinkable character, a warmer temp can help bring out the overall flavor, which helps counter-balance that.
I agree with Jordan, anything malty is a good candidate. I haven't done an Irish red (yet), but when I do, at least half will go on nitro.
Any stout is good, of course. Last winter I had a big imperial stout on nitro, and I wouldn't have it any other way.
I haven't done a barley wine since I got my nitro setup, but it's on my list to brew a big English barley wine and put it up on nitro.