X2: The Threat
It starts very slowly and has a steep learning curve, but if you're patient enough to stick with it, you'll probably find that X2 is a diamond in the rough.
XIII has a unique and potentially interesting premise, but the game doesn't really differentiate itself from the wide array of other first-person shooters on the market.
X2: Wolverine's Revenge
It's certainly not the worst X-Men-licensed game, but Wolverine's Revenge is a game only the biggest fans of the comics could love.
Xtreme Air Racing
Xtreme Air Racing is an innovative and exciting cross-genre game that should keep computer-game pilots and racers busy for a good long while.
On its own terms, Enforcer is a fast-paced and rather fun shooter, though it'll prove too easy for veteran players.
X-Plane is essentially a rigorous flight simulator that simulates everything from helicopters and small civilian two-seaters to jet spy planes and the space shuttle.