I personally find that I do better in cars that suit my driving style rather than the "meta" pick. I can't stand how the Supra and X GR3 drives, but they're more often than not strong picks. If that applies to you as well, it's very hard for someone else to recommend you a car as "the single best".
What you can do if you're tight on money is to go to Daily Race B/C (there's usually one Gr.3 race among the two) and rent a Gr.3 car for the qualifying time trail, and just take it for a spin. Granted, you won't be able to choose the track, but it should still give you a good idea of how a car drives, how well it suits you, and its strengths and weaknesses. Or, heck, sign a manufacturer contract and get loaned a car that you can use in the single player campaign if you haven't already for the season (just be aware that you can't change your manu for the rest of the season).
That said, some of my favourite Gr.3 cars thus far are the 911 RSR and RCZ Gr.3. They're slow as heck on the straights, but are SO confidence inspiring theough the twisty stuff.