In no particular order:
Splinter Cell/Splinter Cell: Pandora Tomorrow I know these games aren' t exclusive to the Xbox (although they should be, IMO), but they' re certainly best played on said system. Two of my favorite games of this generation by far, and both kept me fully entertained from beginning to end. If you want the ultimate in realism (or as close as you can get from a video game trying to imitate elite military operations), you HAVE to play these games.
Deathrow Overlooked and under-appreciated, Deathrow surprised the crap out of me. The action is fast and fluid, and the game is just oozing with style and its much-needed counterpart, substance. A full season is both rewarding and challenging, and multi-player is a blast as well. A must-have for any Xbox owner, IMO.
Dead or Alive: Xtreme Beach Volleyball Go ahead, laugh it up. I don' t care. The game was designed to be a " leisure/relaxation" type of title, and that' s exactly what it is. After ripping about 50 songs from 30 different CD' s into the Xbox, I kicked back and had the time of my life playing volleyball. Sure, it' s a little flawed, but I found it to be great fun. Heck, I still play it from time to time, and I' ve probably logged over 70 hours total. Fun, fun, fun.
Chronicles of Riddick IMO, the best game I' ve played to-date on the system. Yes, better than Halo. Yes, better than SC: PT or Ninja Gaiden. It' s really THAT good. The design, gameplay, story, heck; the entire PRODUCTION is just glorious. Yeah, it' s too short, but it' s certainly worth playing more than once. I just can' t find as many glaring flaws as I can in NG or Halo...as if those games have a lot in the first place.
Ninja Gaiden The best action game of this generation, although I will admit that I enjoyed Devil May Cry more. The extreme level of difficulty, somewhat bland environments, tricky camera, terrible voice-acting, and annoying enemies all kind of detracted from the experience just a tad, but ALL of them are minor complaints. In the end, NG is just an unbelievable title that is near-sheer perfection when it comes to gameplay.