Straight out of the box, I would have to suggest the Fly Lola B98 as being up there with the quickest. The Motorola car has the Fly Racing motor, which makes it very quick, but even the cars with the standard motor go very well straight out of the box.
As for the reliability angle, I would agree with Jamie about Scalextric cars. As for the most fun...a very tricky one, that. Maybe Scaley NASCARs? Plenty of colour and movement and very damage-resistant!
Mooster