Flat keys for anything really. Sharps just seem so... not nice... I guess it's just because I've been playing music that has had up to five flats in it, that I just naturally "attract" toward those keys. One of my favorite piano pieces I have written has six flats in it (then again, it goes into E major, with four sharps! ). BUT, when I do need it, I will write something in D or A major, or B minor or something.
Funny, but when certain accidentals are marked as flats - Ab, Db etc., I can't seem to read them as readily as if they were marked sharp - G#, C# etc. When they occur, I have to convert them mentally into sharps before I can deal with them.
The opposite is also true to an extent. I have to 'think' notes like E#, A# and D# into F, Bb and Eb.
I wonder if it has anything to do with my being a synaesthetic learner.
Definitely flats! However, there are a few notes that I don't like in the "flat form". For example, I can't stand Db's but I like C#'s. Also, I like F#'s not Gb's. I guess it's just a clarinet thing or just a me thing.