I have a mix of all the drivetrains, but although each of them have their merits, I'm going to have to buck the trend (and risk sounding like an utter noob) and go with FF.
To me, they present the greatest tuning challenge (namely their lack of cornering ability with Comfort tyres) and are the most rewarding cars to drive hard and win with. A well-tuned FF can be deadly and outhandle a lot of RWD cars. Plus for the beginner, FF cars are the friendliest,
On the others:
FR - Best for balanced handling. Second favourite after FF. Used to hate them in earlier games as all they seemed to do was spin (I was only 10 or 11 at the time and simpler physics then) but then I manned up and realised just what fun they are - my GT4 garage was predominantly FR. Plus I love shredding rear tyres.
MR - Love/hate relationship. They win hands-down for agility but at the same time some MR cars are twitchy (read: 512BB) and I hate feeling like a wuss for struggling to tame them without TCS.
RR - Great fun with the slow machines (500, Bug, Kubelwagen etc) but I find myself losing patience with overpowered RRs. Saying that, I have several as there's something a little addictive about their on-the-knife-edge character.
4WD - Depends. In some cars I find it creates un-necessary understeer. However you can't beat the superior grip even with rubbish tyres.