So I got Horizon yesterday, the first car I bought (when I was allowed to choose what I want) was the 1971 KPGC10 Skyline 2000 GT-R. I started off by putting an R35 engine in it with some sports tyres and... That was it, and unsurprisingly I couldn't keep it going in a straight line, so I swapped it for the stock S20 and put race cams and flywheel in it instead, reverted to the stock tyres and was left with an awesome little drift machine.
By the end of the day I'd found all the upgrade signs so I decided to test all different configurations of engine since I could do it for free, a fully upgraded NA stock engine is probably my favourite in terms of driveability and sound.
Then I bought an MX-5 and maxed that out with a turbocharged RX-8 engine and an AE86 with over 1,000 bhp, both insanely fast machines, the MX-5 is actually in the R2 class now! Then I bought the Venom GT... And I've just realised, I bet that's a showcase car. Ah well.
But yeah, I'd say Horizon is the best open world racing game I've ever played.