My list in order 1-5
Dead space 2 - That scene in chapter 5 when you fight the brute and it chases you through the hallway. DAMN, I had to put my controller down and think about what just happened after that. It was just so well structured in terms of gameplay to cutscene cinematics, it sent shivers up my spine...
Bioshock 1 - Story line was insane, game was aesthetically ridiculous for 2006, gameplay was unbelievable and the added plasmids and whatnot were freaking insane when i first picked this game up.
Pokemon sapphire/ruby/emerald - Definitely my favourite pokemon game(s) just in terms of gyms, pokemon in the game and childhood nostalgia
Modern warfare 2 - My most played multiplayer game. didn't like black ops 1 or mw 3 so I played this game for like 4 years. 35 days gameplay on it and I still enjoy it.
Gears of war 3 - Mostly for the wallbouncing and sick kills you get in multiplayer. Also i swear theres no better feeling in gaming than wrecking the whole enemy team wallbouncing around popping domes with the gnasher