By way of GuerillaBlack at TXB.
Majority of these titles will release by 2007 or are already out, making X360 the must have console. Some, of course, won't make an appearance until 2008. :-)
Do not quote unless modified. Key:
Also on other consoles - italics
Xbox Live Arcade - * Japanese RPG / SRPGs 
Blue Dragon
Cry On
Cavia RPG
Culdcept Saga
Enchant Arms
Eternal Sonata
Infinite Undiscovery
Lost Odyssey
Magna Carta 2
Operation Darkness
Saga of Cathay
Spectral Force 3: Innocent Rage
Spectral Diario
Super Robot Taisen XO
Western RPGs & MMO
All Points Bulletin
Elder Scrolls IV: Oblivion
Elveon
Fable 2
Final Fantasy XI: Treasures of Aht 'Urhgan
Golden Axe
Huxley
Kingdom Under Fire: Circle of Doom
Marvel Universe Online
Mass Effect
Obsidian/SEGA RPG
Project Gray Company
Phantasy Star Universe
Stargate Worlds
The Lord of the Rings: The White Council
The Precursors
Two Worlds
Real Time Strategy
Battlestations: Midway
Command & Conquer 3: Tiberium Wars
Halo Wars
Star Trek: Legacy
The Lord of the Rings: The Battle for Middle Earth II
Third Person Shooter / Stealth Action 
Army of Two
Bullet Witch
Cipher Complex 
Crackdown
Dark Sector
Data Fly
Dog Tag
Gears of War
Ghost Recon Advanced Warfighter II
Hitman Blood Money
Kane & Lynch: Dead Men
Lost Planet: Extreme Condition
Rogue Warrior
Sneak King
Splinter Cell: Double Agent
Splinter Cell V
Stranglehold
The Club
The Darkness Action / Adventure
Avatar
Assassin's Creed
Captain Blood
Demons of Mercy
Dirty Harry
DOA Code Cronus
Eragon
E-D-F-X
Fantastic 4: Rise of the Silver Surfer
Gauntlet*
Grand Theft Auto IV
Heat
Infernal
Just Cause
KnightBlade: Line Of Defense
Disney's Meet The Robinsons
Monster Madness: Battle For Suburbia
N3: Ninety-Nine Nights
Naruto
Ninja Gaiden 2
Overlord
Pirates of the Caribbean 3
Project Progressive
Saint's Row
Samurai Warriors 2
Schwarzenberg
Secret Clan
Shrek the Third
Star Wars 2007
Superman Returns
Tenchu 4
The Conveni X
The Outsider
The Siege of Khe Sanh
The Shadow of Aten
The Simpsons
Thesis
Vampire's Rain 
Too Human
Way of the Samurai
Survival / Horror 
Alan Wake
Alone in the Dark: Near Death Investigation
Clive Barker's Jericho
Condemned
Dead Rising
Harker
Heavy Rain
Possession
Resident Evil 5 Fighter / Wrestling
Dead or Alive 4
Def Jam 3: Icon
Fatal Fury Special*
Kengo Zero
King of Fighters: Maximum Impact 
Small Arms*
Senko No Ronde Rev. X
Street Fighter II' Hyper Fighting*
TNA iMPACT
Ultimate Mortal Kombat 3*
Wrestle Kingdom
WWE Smackdown VS RAW 2007 Mech Combat / shooter 
Armored Core 4
Chrome Hounds
Mobile Suit Gundam
Mobile Ops: The One Year War
Project Sylph
RoboBlitz*
Zegapain XOR
Racing / Car Combat
1XR
Avalon
Burnout 5
Burnout Revenge
DIRT: Colin McRae Off-Road 
Forza Motorsport 2
Grand Raid Off-Road
GTR
Moto GP
Need for Speed Carbon
Project Gotham Racing 4
SEGA Rally Revo
Stuntman 2
Test Drive Unlimited
The Wheelman
Tokyo Extreme Racer
Platformers / Party 
Banjo-Threeie
Bomberman Act Zero
Cloning Clyde*
Every Party
Fuzion Frenzy 2*
Indiana Jones
Kameo Elements of Power
Ratatouille
Rayman Raving Rabbids
Sonic the Hedgehog
Sonic the Hedgehog High Speed*
Spiderman 3
Teenage Mutant Ninja Turtles
VooDoo Nights
First Person Shooters
Battlefield: Bad Company
Bioshock
Brothers in Arms 3 Hell's Highway
Call of Duty 3
Cell Factor: Revolution
CrossFire
Doom*
F.E.A.R. 2 
Halo 3
Half-Life 2: Episode Two (Episode 2, Portal, Team Fortress)
Haze
Hostile Intent - Planetfall
Interstellar Marines
Killing Day
Medal of Honor: Airborne
Parabellum
Perfect Dark Zero
Postal 3
Prey
Project Offset 
Rainbow Six: Vegas
Return to Castle Wolfenstein II
Section 8
Shadowrun
Timeshift
Turok
Unreal Tournament 2007 Simulations AquaZone
A-Train X
LMA Manager 2007
Outpost Kaloki X*
The Idol Master
Viva Pinata
Sports
Amped 3
2K Sports Football game
Brian Lara International Cricket
Cabela's Alaskan Adventures
Championship Manager
College Hoops 2K 
DOA Xtreme Beach Volleyball 2
FIFA
FIFA World Cup Fight Night Round 3 
HoopWorld*
Madden NFL
Major League Baseball
NBA Live
NBA 2K
NBA Street Homecourt
NCAA Football
NHL
NHL 2K
Pro Evo Soccer
R* presents Table Tennis
Skate
Tiger Woods PGA Tour
Tony Hawk's American Wasteland
Tony Hawk's Project 8
Top Spin
Virtura Tennis
World Championship Poker
World Snooker Championship Music / Rythm
Dance Dance Revolution Universe
Guitar Hero II Special Projects / Other
Fantasy Lab project
Konami Untitled Project
Peter Jackson Halo project
Untitled Treasure Project
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Will try to keep updated.