If you don't count the hybrids, it'll be the GTOne at a little over 345mph.
The hybrids run whole lot more than the "Watch Race" button will let you see, as for some reason this maxes out at 357mph. (Same for replays of cars you weren't driving)
Hybrid top speeds:
X2010 - 1049mph (readout maxes at 999mph, but the steering wheel readout shows 1674kmh)
Delorean - 603mph
Peugeot 307cc - 393mph
Honda Civic - 363mph
Formula Gran Turismo - 355mph
High End G37 - 279mph
Kart 100 - 172mph
Also, search is your friend -
Top Speed of GT5 Cars
(Hybrids are not going to be listed in the speedtests of this thread)