To be honest, there really isn't many "must own" titles besides Halo out for the system right now. Here are the ones I have:
WWF Raw - AWESOME graphics, okay gameplay. Not many game modes, but if you are a wrestling fan, this one gets better the more you play it, I give it a 7/10. Fairly shitty create-a-wrestler mode, but the entrance creation is a blast. Fear the Crossface
Madden 2k2 - Old habits die hard. Best version since '94 with tough revamped D. Go Browns! 9/10
NFL 2k2 (Sega Sports) - Another great f-ball game, graphics not up to par with Madden or Fever, but a GREAT game to play head to head with friends. Top notch announcing and passing control. Realism is superb. 8.5/10
UFC TapOut - Anyone who played this on the Dreamcast can vouch for the quality of this title. Great graphics, good controls, shitty x-box controller. Great game to get drunk too. Ice T as a hidden character is stupid. Get Lemmy instead next time please. 7.5/10
NBA 2k2 - Best basketball game ever for any year or platform. This is coming from an old die hard that can't stand the b-ball scene in its street ball persona. Most fun I have had with a b-ball game in awhile. Not to mention the first one I bought. Graphics are passable, but the control and the AI are unsurpassed. Fantastic announcing also. 10/10
Halo - Speaks for itself. 100/10
DOA2 - No virtual fighter 4, but graphically stunning and another great beer game. Show off game for when unknowing friends want to see what the craze is about. Gratuitous sweater meat as well. 8/10
I also had Wreckless: The Yakuza Missions (5/10), but I traded it in to help pay for Jedi Knight 2 on the PC. It's a rental at best, I bought it "blind". Mind numbing graphics, repetive gameplay that gets B O R I N G in a hurry.