The worst car is the F550 of course, the ultimate n00b car. It's not fun to drive at all.
For fun you should drive the 993 GT2. It handles like a beast, and it sure cannot beat any of the other GT's and the Corvette, but it's fun. Very bad fuel economy gives you no chance to win a race at all, also because of this it's diffficult to race against the 911 RSR in longer races.
How to drive a 911 GT2, you should care for the right gear ratio's, which is difficult choosing sometimes as there are only a few available. A nicely handling 911GT2 is for me 2 splitter, 9 wing. And on faster tracks 1 splitter 3 wing. Some higher rear ARB 5 or 10N. Further the best compound is medium/medium. The Dunlops a re bad tyres, and soft rear is overheating, hard rear is also overheating, this is because of sliding. medium is okay. Front: Soft for short stints, medium usually, never hard.
And yes, because of the engine in the back you need a different driving style. Some trail braking and late on throttle are the keywords. All the 911's have it, but the 911GT2 is the most difficult car to adapt in GTR2 I think. Only the 911 from GTL is trickier.
Important things a bout a 911:
Never brake in the corner, (the rear will overtake the front).
DO not throttle too much in the corner, as you'll understeer like hell, tho with a good setup (more ARB, more caster, more front camber and less front ARB, harder rear springs, softer front springs.), it's a bit cureable.
The Seat is a car from which you have to learn it's tricks when on a trickier setup. And than it's possible to get near the M3's...