I'ts hard to say absolute best cars, maybe over time it will be more clear. For e.g in 450PP most people were using Elise 111R, I went online on Deep Forest (this was months ago) and some others were in the Elise, I was driving an MX-5 flawless, neck on neck with an Elise 111R, I overtook him and thought I had 1st place, and little did I know (until the last minute when I saw the map) some Brazilian guy was racing in an MG TF160 and cained us all with about a 20 second lead and shocked everyone lol.
Back then I bought the TF160 and realised how much it owned

Same with 500PP there are some seriously fast cars: Ferrari Dino, Elise, Focus RS, Civics, Integras, Mazda RX500, Honda HSC, I thought I'd seen most of them then someone owned the hell out of all of us in a Dome Zero.
At the end of the day there are quite a lot of fast cars, some more suited to some tracks than others, some might dominate on twisty tracks like Autumn Ring, some might be good on medium like Deep Forest, other on tracks like Nordschleife.
A lot of cars from the 60's and 70's are very fast because they are very lightweight, have good acceleration and lots of oversteer, and normally have low PP for how good they are, as well as they are normally aerodynamic.
This is mostly all on RS tyres racing online, but of course they will still be extremely fast on any tyres, but SS and SH does open it up a bit more and give some cars more of a chance, giving even lesser cars more of a chance as consistency plays a bigger part of the race.
If only there were official in game leaderboards it would be much easier to see exactly what cars dominated at what PP on what tracks, and how I would love to try and rank up on that, like back in the Forza 2 days, tweaking, hotlapping non stop. GT feels more like a driving around game at the moment, than a competitive racing game. We have the seasonals but they are nothing, there are no proper structured leaderboards on any track / car you like, online most people want to cruise / race mixed PP / race on RS, sometimes allowing SRF, it's a random mess. There's no thing like what Forza had where it was basically "look, it's a class B race, just race" best lap would be put on a global leaderboard and races were consistent. Everyone could clearly see what assists you use, and what PP your car was.