Beepster
It would probably be frowned on by XLN but JamStix (or whatever it's called) would be pretty darned useful to have onboard. Just the algorhithms. Not necessarily the sounds. I'd use it with AD or BFD. I miss just jamming out with drummers and that seems like about as good as it gets in the virtual drummer world.
Mind you I do like ADs beat/swing/feel picker thingie. It's not the same but with a little more effort it seems to have some decent jam out possibilities.
I do this with Jamstix all the time. You can hook the MIDI out of Jamstix to any drum VST you like. Works great with AD, BFD, Battery and everything else. It's awesome!
Personally I wish Ralph would hurry up with Jambassist, which he's apparently been working on for years.