Of the two Monkey Island games, I prefer the original. It always felt more playful, had a better atmosphere (not as "silly" as the second), and more logical puzzles. Number two was way too difficult for me - I had to use a walkthrough to complete it. Still, it was a great game with some very good moments (the cemetary on the first island, for instance - both the digging part and the part where you revived the dead cook).
Both were great adventures, though. Shame they make so few of them these days (Syberia is an excellent adventure, though - and there's a sequel coming, which I really look forward to).