Hmm, a lot of the cars I've driven make aproximatly the same amount of power. It was probably a '92 Silverado, but I don't know which engine it had, so it could have been 175 or 210hp. If it was the 175hp version, the most powerfull car I've driven could be a Taurus (2003) I rented, but I don't know which engine that had either, it could be 155 or 200hp.
If I drove the 175hp Silverado and the low-power Taurus, it could also be a rental Pontiac Montana. It had the 3.4l LA1 V6, which made between 170 and 185hp.
Also in the running is my Dad's '89 Pontiac Safari (exactly like this one. I know it had a V8, but I have no idea what kind or how much power.
I think the first car I ever drove (my Dad's Chrysler LeBaron wagon) had a 360cid V8, but it was made in the late 70s, so I wouldn't expect it to have very much horsepower (lots of torque though).

Sadly to say, since I have only been driving for the past month and a half, and therefore have only driven about 6 different cars, the fastest one I've ever drove was just my driving instructor's 2002 Oldsmobile Alero

Past weekend the Corvette Driving Days took place in my hometown. I was able to drive three different Corvettes. C6 Coupé, C6 Convertible and the 505-HP-Z06. As yet the Z06 is the most powerfull car I ever drove (the black one is the Z06):