The "Best" is very subjective.
What are you going to do?
Drift, keep it stock or light mods for photos, endurance racing, championship racing, credit grinding, having a fun race?
There are some cars in the game that you may use for 1 event only and never use it again.
Such as the BAC Mono for that 1 endurance race.
The MX-5 Touring race is fun, you might be able to use that car in other events, but its low on power.
The Suzuki Swift Sport Gr4 with a medium turbo is great for the WTC700 at Circuit De La Sarthe.
Unless you do a bone headed move like not pit for wets.
A win is nearly 100% so long as you keep the fuel map on 5 and when pitting add enough to finish the race based on time remaining/ 4.25min per lap = laps of fuel to add,
Only weakness is it being FF and getting understeer when cornering and accelerating out of corners.
If you want a bit more of a challenge for that event.
The Alpha 4C Gr4 with a large turbo.
The car needs some work to deal with lift-off oversteer, a win is doable but you will need to use fuel map 6 the whole time as well as short shifting and lift & coast,
Even though your in Fuel Map 6, the car can hit 280km/h.
The GTO Twin Turbo '91 is decent and can do the Kyoto 1hr Endurance.