Two favourites:
1. Lotus Esprit (the premium one). Fell in love with it when I used in one of the seasonals at Nordschleife (British lightweights?). It's great fun to drive, especially in cockpit view

And most of all, when you floor it you can really feel the car accelerating!
2. McLaren MP4-12C. Great looks, definitely one of the best looking cars in the game, but the main reason for being a favourite, however, is the small display to the left of the steering wheel that shows in game clock (so you see what the time is at the current track!), odometer (that accumulate every single meter you drive, regardless if it's free run, online or offline... so you see the true mileage you have driven!) and trip odometer! It's just brilliant.
Wow, that race mod looks hot! I need to get it