Best all around IMO is the F-1500T/A. It won't win every race for you but it'll get you in the running to win some main and post game content.
UNLESS you have the 25th Anniversary Edition. In that case you already have a great car in the Living Legend. Use it wherever you can.
Really it depends on what you're wanting to spend and if you can muster it up on command. If you're looking at, say, half a million, you could tune a road car.
At some point you'll need a good mid engine road car, that's the Citroen and invites for it show up fairly regularly. You'll also want a Porsche and a Ferrari of some type for specific events, the Living Legend and the F-XX-K will decimate the competition and pick up where the Citroen road car leaves off.
Late game, the Red Bull Standard and Red Bull Comp are very potent. I also like the Porsche 919, but like the F-XX-K, its KERS is limited (this limits the 919's four wheel drive too).
Unlike real life, you can't go wrong with anything Ford (IRL that only applies to their trucks). The Raptor is probably the least suited Ford for GT7 but it can still hold its own. Any GT40 variant will work well.
Some other good choices were listed above. The Sauber Mercedes is always a fast opponent and the 787B is a fine car by any standards.
My choice cars through the years have been as follows:
GT1: 1995 Impreza WRX, Dodge Concept Car.
GT2: Ford GT40, Vector M12 (detuned in some cases).
GT3: F 090/S, F 686/M, Eunos Roadster.
GT4: 787B Road Car (all black).
SCUD Race: 1995 McLaren F1.
1998 Sega Rally: the Stratos, what else??
