I haven't driven so many cars (about 10 cars) in my two years of having a driving-license - but here are my both favourites for best and worst:

Best car:

Mercedes Benz SLK 320 (6 speed manual)



The whole car is just amazing! I love the V6 engine and the manual gearbox was also extremely great to handle (only the 6 speed manual of a Golf GTI is miles ahead)! The handling of the car is also absolutely amazing - you turn the steering wheel and the car goes the way you want...
The SLK is a great cruising machine (you can drive it in 5th in the city), but you can also have a lot of fun with it at the autobahn (It reached 260 km/h - although the official top speed is 245 km/h)!
The fuel consumption of the SLK 320 isn't bad at all (average of 9 to 11 litres / 100km)!

edit: Will add some information about the second best (Jaguar XJ8 3.5) soon!



worst car:

Renault Modus (1.5)

I drove this car, when our 1998 Renault Megane Scenic was at the service! I really hate this car - the design is absolutely horrible and the driving experience was exactly the same!
The car feels unstable in curves and the steering was synthetic. I wasn't able to see the numbers on the digital tacho, when the sun glared into the car.
And also the engine and the manual gearbox don't really facilitate the "fun and comfort" of driving a car!