Obviously all of them, but Panzer Dragoon Orta, Jet Set Radio Future, OutRun 2, and the new Virtua Fighter if it ever arrives would be great. Sega put out some of their best games ever on the original Xbox, but emulation for that console is still underdeveloped and their 360 backwards compatibility was never perfect if I'm not mistaken. If I'm being honest, I'm just not really an Xbox person so it's a hassle to haul that thing out of the closet and plug it all back in. Those three are literally in contention for best Sega game ever.
New Virtua Fighter needs no explanation. For me, I don't feel as much need for a lot of the games because I either have them handy or they emulate more easily, but I do think there are some big names and franchises that should be better preserved, many of which you mentioned in the OP.
I think now that Saturn and arcade emulation are getting to the point they are, it's time for games that didn't get their fair chance the first time around like Golden Axe: Revenge of Death Adder, OutRunners, Daytona USA 2, Shining Force III Trilogy, and so on to be getting their due.
