What is your favourite fantasy RPG? System which it can be played on is irrelevant, and MMORPGs sort of don't count, since they're entirely "Grind, Grind and Grind some more"
And a reason or two would be nice.

My favourite, for sheer characters alone, was Baldur's Gate 2...
The characters in that were completely unforgettable, and the storyline wasn't the best out there, but it sort of drew you in with the completely vibrant (if quite isometric) world. You tend not to forget Minsc and his humourous non sequiturs, Viconia and her acid wit, or even Jon Irenicus' apathy and amorality which made him such an excellent bad guy (if a bit monologue-y).