I have listened a lot to his preludes and they are all wonderful compositions. At this moment, I like the B minor best. Probably because I know the story behind it (told by Benno Moiseiwitsch in "Art of the Piano").
I've seen that and I agree, the B minor prelude is quite a piece.
While it's hard to pin down an exact favorite, here are my top ones from the entire set of 24:
23/1 in F sharp minor
23/2 in B flat major (wickedly difficult, I find very few pianists can play this piece well)
23/3 in D minor
23/4 in D major
23/5 in G minor (maybe a bit overplayed, but I have always liked the central section of it)
23/6 in E flat major (this piece is like being in a dream)
23/7 in C minor
23/8 in A flat major (such an atypically happy and playful piece for Rachmaninoff)
23/9 in E flat minor (another one of the few pieces I find totally difficult)
32/1 in C major
32/5 in G major
32/6 in F minor
32/12 in G sharp minor (a favorite of many, I'd like to play this someday)
Here's a nearly flawless video of Lugansky playing No. 7 and No. 5 from Op. 23 (as well as an awesome performance of Moment Musical No. 4): http://www.youtube.com/watch?v=-iYhP5m3M4E
I'm probably going to get a lot of flak for saying this, but I personally have never really cared much for the C sharp minor prelude (op. 3) and don't really understand much of the fascination that surrounds it. Even Rachmaninoff himself found the constant stream of requests from audiences to play it bothersome, and I read somewhere that he'd even make up excuses not to play it, playfully telling the audience that he'd conveniently "forgotten it" (what a character!). While I agree that it's probably one of the few pieces that launched him into fame as a composer, in my opinion he wrote volumes of way better material that make that piece seem almost trivial.
The reason I really like 32/1 and 32/6 are because the style in which these pieces are written foreshadows that of his etudes, which I think are some of his most amazing piano works.
Among my favorites from his etudes are:
33/2 in C major
33/4 in D minor (this is seriously such a cool piece, I'd really love to play it someday)
33/5 in E flat minor (I nicknamed this one the "whirlwind etude," as it reminds me of Chopin's "winter wind" etude)
33/6 in E flat major
33/7 in G minor (very sad, deep)
33/9 in C sharp minor
39/1 in C minor
39/3 in F sharp minor (I love this piece!)
39/4 in B minor (Rachmaninoff himself loved playing this piece)
39/5 in E flat minor (My piano teacher is teaching one of her students this piece now, it's beautiful)
39/8 in D minor
Sorry if it's too off-topic, but I kinda have a thing for Rach's music.