Gr.X: F1500-A
Gr.1: McLaren Ultimate VGT
Gr.3: Porsche 911 RSR
Gr.4: BMW M4(any version)
Gr.B: WRX Gr.B
Ever changing BoP and handling models have shaken up the must have cars per group over the months. As of October, what you want is;
Gr.X: The Mercedes F1 car has supplanted the F1500 as the main formula car in the game. You need both for GT League though, and as for Sport Mode, no one Gr.X car comes up that often.
Gr.1: One of the three 2016 LMP1 cars. All three looked competitive at Fuji and they're never a bad choice for any Gr.1 race. The McLaren could still be good at Le Mans or Monza, but has lost it's shine everywhere else.
Gr.2: Player's choice. The Honda is tricky to drive but has the best cornering of the trio. The Lexus and Nissan are safer picks and have the slight edge down straights.
Gr.3: Porsche 911 is still good on technical circuits, though I feel the Mitsubishi is easier if you're using a DS4. For power tracks, the Ford GT is often the outright fastest, albeit suspect on tyre wear. Good all-round picks are the Mercedes AMG GT, the 2017 Lexus, and the Aston Martin.
Gr.4: Gets tricky here because the BoP is a mess. The Porsche Cayman is usually okay if not good at any given track though. If tyre wear is off, the Audi TT can be dominating on some tracks. The MR Megane Trophy used to be insane but got nerfed hard and only works on very technical tracks now.
Gr.B: The Audi Quattro ends up being the overwhelming favourite on the rare occasions this comes up in Sport Mode. Might be a different story on gravel, but gravel never comes up, so the Quattro's handling problems are masked. I've seen the WRX compete reasonably well though.
Keep in mind that I've focused more on Sport Mode here. GT League by contrast has no BoP. So far example, the outright fastest no-BoP Gr.3 cars are the Dodge Viper and Volkswagen Beetle, whilst the Mustang and Nissan GT-R are the slowest.