What car should I buy???

If you are looking for a competetive sportscar i'd recommend the Feltzer or any of the new racecar versions of Massacro and Jester.
Incase you dont have the Zentorno, that would be a great investment.

Don't know if you're only referring to sports class but I'd say 1 of the 2 new racecars, or Feltzer if you already have both. Jester will dominate on a tighter track, while 'top-end' heavy tracks will favor the Massacro imo.

You have three of the four best sports cars in the game (only missing the Feltzer to complete that set). If I were you, I'd look at branching out into other classes. Supers are obviously faster, but much more expensive if you're strapped for cash. How much money do you have/are willing to spend? That'd help us to help you.

If you want another competitive car, but in a different class, you can pick up the Dominator from off the street and add that to your garage. Same for the Zion, and the (much rarer) Schafter.
Try doing some races on your own with different cars that take your fancy, and see which ones you like, and which suit your style. That's how I ended up choosing the Pigalle over the Coquette Classic.

Of course, you might not want a car that is best used in races. In that case, get something a bit more fun, like a Comet.

Hope I've helped you in some way.

 i have already the  simple jester... Interests sell my old jester and buy the new???

You should get yourself a Panto.

No what you need my friend is a lawnmower.

No you get the surfer for van races because its awesome.