The Spicy bangers Car Pack is the best one I've played with; lots of cars with lots of customization and all balanced with the official content so nothing is too weak or too OP. Very, very well made mod pack in my opinion; it genuinely feels like an official expansion to the vehicle roster.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2173468119

I also like the Caddie because it's funny (and again fits in pretty well with the official vehicles). However, some of the comments indicate that it doesn't play very well with other mods for some reason, so keep that in mind.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2825489403