Am struggling with that very question now....
Decisions, decisions.
Am tempted by a Provdrive STi Impreza, but if next year's trackday tool turns out to be an Impreza, seems daft having another one on the driveway.
Passat R36 looks good, but have a feeling it may end up being a bit of blamanche, and handling like a fwd car when pushed.
I hate to say it, but bmw is looking favourite with either 3 or 5 series M-sport.