1. Deus Ex
Simply due to patience required and lack of autosave.
2. Halo 2
Beating this on Legendary is no easy feat.
3. Far Cry
Weird level design and too-perfect AI.
4. Operation Flashpoint: Red River
One bullet kills.
5. System Shock 2
Like Bioshock, except much less forgiving.
Difficult puzzles and a super-convoluted final level.
7. Tom Clancy's Rainbow Six: Vegas
Like Deus Ex, requires more thought than twitch reflex.
8. Call of Duty 4: Modern Warfare
Infinity Ward is not afraid of grenade spam.
9. ArmA II
Uber-realistic. Death is very easy.
10. S.T.A.L.K.E.R.: Shadow of Chernobyl
Irritating level design, weird A. I.