I've been replaying Marathon(s) 1 and 2 through Aleph One recently. I was there when A1 became a project in the Marathon community and it's so cool to see bungie embrace it as their de-facto way of keeping Marathon alive.
Hell of a series, still. The gap in quality beween 1 and 2 is kind of shocking (though I've a ton of love for M1 since, you know, she was my first), but M2 still feels weirdly original. Doomlike maze levels is one thing, but the integration of puzzles and narrative really slam home how ambitious Bungie were.
I'm still torn on Marathon Infinity. A non-linear parallel-universe exploring mindf**k of a game, and the best technical use of the engine, but somehow it just doesn't come together like it should. The first level alone is a colossally confusing mess of claustrophobic corridors and unclear objectives and it kind of just gets worse from there.
Regardless, these are stellar games IMO. If you've never played Marathon, I think you should check it out. M2 in particular is a stone cold classic.