Speaking for myself:
Slow road cars are boring for hotlapping.
Computer assisted road cars (MP4-12C, any Skyline) are boring full stop, in reality or simulated.
Cars that are difficult to drive are most fun for hotlapping.
With multiplayer (which, for me, is AI, as I don't have much interest in racing random people of dubious intent) then the whole thing changes, and it's less about "the car" and more about "the race". So the slowest cars in the world often provide FAR better racing than anything quick or difficult. UF1000 racing can be tremendous fun on the right circuit.
Hopefully when multiplayer is added, there will be sufficient interest and sufficient options to enable me to play against people I know with a similar mindset - I don't want to play against people that aren't as serious as me, or are too willing to utilise any unrealistic advantage (e.g. LFS's old ride height aero glitch), because that's not fun for me. The best online racing, since LFS S1 and the early S2 days, was in nKP when it came out, because only the nerds were playing it.