Any one with the highest and most balanced performance I can afford, in OE or modified condition. In GT4 I started out with the VW Lupo Prize car from Licence.
In the past, a zippy "frontwheeler" with dual engine tuning cabilities (N/A and charged induction) and managing of prize cars have been the quickest path to in game advancement. This applied no matter if the agenda was game completion, to unlock certain tracks/challenges and/or building the perfect garage. I expect the same will apply for GT5.