by best i mean:

1) Stable
2) Compatible (MP3, Vorbis, AAC)
3) Any aditionals

I've only had the chance to lay my hands on BPM Studio and Traktor (and that was some time ago at a small radio station).

Both are fine pieces of software. I believe they target a completely different audience. My definition of DJ is cool guy at the decks of your neighbourhood trance club with beat matching skills. Traktor is for him. The "other" type of DJ who sits in a radio station and plays music without beat matching and genius transitions is the person who would be using BPM Studio.

BPM Studio can also do beat matched mixing and surely Traktor will allow you to just mix without it, but it's a matter of usability and features.

AFAIK the latest Traktor version can read FLAC and AAC (but not Vorbis).

Do any of these support mpc? I've given up looking, asking for a new codec is like asking for a bit of the moon on some forums. Or alternatively do any support any directshow codecs, so I can use mpc through the radlight filter?

my vote goes to Traktor DJ Studio, but version 1.06

latest traktor supports: DRM-free WMA and AAC file formats are now supported, as well as Ogg Vorbis und FLAC (lossless audio compression).

A few people have been having problems with AAC files in Traktor 2.6, me included!

Reports of buffer garbage appearing at the front of the wave file and beat markers moving.

Have a look at this open source mixing software:

http://mixxx.sourceforge.net/

Unusable at the moment in my opinion, but I think it has potential.