Err... this is hard. 5 top games that I mostly would name, can't get them into any order though (and I'm counting a few franchise rather than individual titles):
Half Life franchise
System Shock franchise
Deus Ex (pity I can't say franchise)
Grim Fandango
Thief franchise
basically I prefer a first person perspective for immersion but I want a compelling atmosphere and narrative with some light RPG elements - with the exception of Grim Fandango, which I'll take in any perspective I can.
Try to be reasonable... its easier than you think...