Ever since it was added to the game the Issi classic has easily become a fast favourite of mine. Granted there is a slight bias as it is a car I aim to own irl but it's just so much fun to drive I love it in GTA too.
I still regularly drive the Coquette BlackFin (topless) for relax driving and to chill around. For racing I mostly use Pariah/Vagner for performance, but based on visual and sound, XA-21 is also my favorite with its grunting noise, really close with the Visione for its Mclaren P1 style.
The Pariah in real life is amazing. (AM Vanquish Zagato) This car will win you any race you hop into with randoms. I also agree, Dirt Rally Racing in this game is incredibly fun (Lucky Jim's) especially with cars like the Flash GT, Omnis, and Sultan RS. For supers it is the Vagner. Muscle is PissDom. Motorcycles is Shotaro or BF400 on long straight tracks. The ultimate assassin, they see you pull up in that BF400 and they never know what's coming to them.
My favorite car inthe game ( non races ) is the Tropos. It's one of the cars that handle the best in the game, close behind it are the tempesta, zentorno, retinue... But nothing can beat the tropos when it comes to driving...
I've even won a race against a Pariah in my trusted Tropos.
As for racing the elegy is probably my favorite. It's fast but not too heavy.