Heres a list of decent XBox Xclusive games. I'm not counting PC ports either, and Shenmue 2 is debatable depending on where you live.
1 - Brute Force
2 - Buffy the Vampire Slayer part ONE
3 - DOA3
4 - Deathrow
5 - DND Heroes
6 - Morrowind (PC)
7 - Ghost Recon: Island Thunder
8 - House of the Dead 3
9 - Jet Set Radio Future
10 - Gunvalkyrie
11 - Mech Assault
12 - Moto GP 1 & 2
13 - Midtown Madness 3
14 - Starwars KOTOR (PC)
15 - Steel Battalian
16 - Crimson Skies
17 - Top Spin
18 - Amped 2
19 - Ninja Gaiden
20 - Otogi
21 - Panzer Dragoon Orta
22 - Project Gotham Racing 1 & 2
Sega GT Online (?)
Future Xclusives that shouldn't dissapoint:
23 - Counter Strike (PC)
24 - Deus Ex (PC)
25 - Conker: Live and Uncut
26 - Jade Empire
27 - Fable
28 - Halo 2
29 - Kingdom under Fire
30 - Otogi 2
31 - Phantom Dust
32 - Perfect Dark Zero (In 2008 knowing Rare

)
33 - Ralisport Challenge 2
34 - Spikeout Xtreme
35 - Starwars: Jedi Academy (PC)
36 - SB: Line of Contact
37 - True Fantasy Live
38 + - Any Rare game, Bioware game, Microsoft or Sega game that you may enjoy.
Theres almost 150 XBox exclusives to name if you don't count the PC games. I'll be working on a list again by the end of December if anyone is interested in it. These are just the games I would buy/give a try in if I had the money.
I already own 47 XBox games, and I will say the third party games that are out on other systems also take up my list. Theres also quite a few good live games that I didn't list. Also checkout gamerankings.com and they'll give you a run down on the top XBox games out there.