Probably my fave writer, and certainly the only one who can reliably bring a tear to my eye and make me feel like I'm having a conversation with the characters he writes - Life After God got me through some very dark points of my life and for me it's still his most emotionally affecting work - he gets loneliness in modern society in a way that no one else does for me.
Fave book would be between Mircroserfs and Miss Wyoming - and controlversially I'm not a fan of Girlfriend...I seem to be the only Coupland fan who doesn't rate it...
Gen X...well, it's a toughie really. I loved it when I read it and still loved it when I re-read it 6 months ago. Everything about it from the jargon ('Options Paralysis') and the slogans ('Use Jets while you still can') for me are as relevant now as they were 14 years ago

, and the stories that Dag, Claire and Andy tell each other still make me smile (I also downloaded a Texlahoma story from his website too...how tragic am I?). Dag's comment about being able to deal with the aftermath of a nuclear war OK but freaking out if he saw a jet pushes buttons in my head for some reason.
But is it of it's time/era...I reckon you have to be a media junkie to really enjoy it, and it's probably the most US culture-specific of his work and yeah, it is very much a book that talks to a very specific group of people at a certain point in lives and the early 90s...
But then I also still really love BEE whom I started reading at the same time (American Psycho and Gen X were published about the same time IIRC) and there are lots of peeps who love/hate the other one...it's interesting to see how they've both changed, and the themes or the books and stuff tho - both explore contemporary alienation and loneliness through very different prisms.
Or something.
Oh yeah, and the first section of Nostradamus scared the shit out of me the first time I read it. Genuinely brilliant descriptive writing and it made Coumbine a lot more real to me.