Split in nostalgia favourites and modern favourites in no particular order (with a small list of reasons):
Nostalgia:
Baldur’s Gate II Soa and ToB (Best RPG dialogues of yet, wonderful team bantering, deep and complex storyline)
The Longest Journey (My first actual graphic adventure, wonderful story, its aesthetics still figure prominently in my design ideas)
Jedi Academy (Spectacular fighting system for its time, first actual MP experience)
Street Fighter II Turbo (First fighter game experience, state of the art game for the time, HADOKEN)
Vampire: Bloodlines (Very immersive and fun, though it could have been so much more)
Black & White (Wonderfully designed and incredibly deep game, still one of the best videogames ever, one of the few games i would buy again if they made an HD remake without changing the gameplay)
Max Payne (Still probably one fo the best shooters of all times, masterful plotline and great execution all around)
Modern:
Bulletstorm (Fun, apeshit, brilliant game, though tainted by out-of-place modern mechanics)
Mass Effect 1 & 2 (Great RPGs, great gameplay)
Persona 3 and 4 (again, great RPGs, despite the shitty standard JRPG gameplay)
Guilty Gear and BlazBlue (Best fighters on the market)
The Witcher 1 (Masterful debut title, based on great source material, very european sensibilities, spectacular all around except for the extremely sub-par animations, probably due to the lack of specialists in the studio)
The Witcher 2 (Same as the previous but with spectacular animations)
Shin Megami Tensei: Devil Survivor (Best RPG on the NDS, deep and complex storilyne plus it’s Shin Megami Tensei)
Alpha Protocol (Not very well coded but innovative approach at RPG dialogue and plot development, pity about the shitty stealth, which is kind of a big downside considering it’s a game about spies)
Spider-Man: Web of Shadows (Best Spider-Man game on the market, pity about the badly thought out multiple endings)
Soul Calibur III (Very intuitive gameplay, sooooo many evenings spent twatting eachother with blunt swords while drinking heavily and laughing like morons)
Dead Rising 2 (Funny, free and original, though bugged up the arse)
Left for Dead 1 & 2 (Best if played with friends, though)