I thought the first Mario Galaxy was just an alright game when it came out, but Super Mario Galaxy 2? This game knocked my socks clean off.
The sequel was 10× more creative and far more tight in its level design and challenge than the original. It's incredible the shear imagination and variety on display. A new, clever gameplay concept is introduced every few minutes. It just never let's up.
There's seriously enough concepts and ideas in this one game to create 20+ full fledged games.
I can't think of another instance where a direct sequel felt more fresh and novel than its predecessor.