For me it’s either Splinter Cell Chaos Theory (lot of different ways to play through, getting stealth rating’s to 100% and just very satisfying game) or Uncharted 2 (linear but amazing campaign, replay value mainly just experiencing those wonderful moments again and again, but also looking for treasures and different difficulty settings).
Call of Duty, United Offensive 2,Modern Warfare,5, Modern Warfare 2 and 3