I liked the Library.
Also, on Chojin's indirect reccomendation, I downloaded all three Marathons today.
Infinity has some mega claustrophobic level design. Also, it's really really dark, and I think this is the game where Bungie introduced their TIME HONORED TRADITION of every room in every level looking exactly the same.
1 isn't nearly as bad with crap like that, but I stopped playing it when I couldn't find my way around the service corridors in the first level, which were again ridiculously tight.
That said, 2 is fucking godlike. I highly reccomend it to all of you!
http://source.bungie.org/get/