Yermo here's a list of some of my favored "track bikes", I'll break it down into three groups; track only (with riding group, these are bikes that would be taken off the street and used as dedicated track machines only), track+street, and Top-Shelf (fastest/hardest/most$):
Track Only
-Kawasaki ZX6R (I and A)
-Suzuki GSXR-600 (I and A)
-Yamaha R6 (I and A)
-Triumph Daytona R (I and A)
-Suzuki SV650 (B)
-Kawasaki Ninja 650 (B)
Track+Street
smaller---
-Triumph Daytona
-Kawasaki ZX6R (636)
-Honda CBR600
larger---
-Suzuki GSXR-750
-Kawasaki ZX10R
-Honda CBR1000R
-BMW 1000RR
Top-shelf:
-Aprilla RSV4 Factory
-Kawasaki ZX10R
-Ducati Panigale
Good Used Bikes for Track+street:
-2006 Kawasaki ZX636
-Honda RC51
-2005ish Suzuki GSXR1000
-Honda CBR1000 (fireblade)
In my own opinion the +/- 600 class makes for a great track platform, plenty fast with good cost containment. For the street I prefer a larger bike, mostly for the better and more usable mid-range that can be far more useful day to day as well as the more forgiving ergonomics.