If you want fun cars...
The Nissan Skyline GT-R V-Spec II is a great Japanese powerhouse. Fully upgraded, it hits the 900 levels, and tears the streets off.
The Nissan R390 has a pretty low top speed (only 340km/h), but is GREAT on the small city tracks. Corners hard, accelerates harder.
BAC Mono is a neat track car. Sticks to the ground like it's magnetic, amazing cornering. Bonus if it's a Horizon edition, for extra credits.
The Holden Torana A9X is the perfect template for a drift car. There are multiple blueprints available online, built specifically for drifting. They start sliding if you only steer just a little bit.
For offroad, the Nomad is great. But if you want some variation and want to try out some rally cars, the Mitsubishi Evolution IX (upgraded) will outrun professional rally cars. The Lancia Delta is great as well.
Like I said, the Ford GT is a great hypercar. Hits very high speeds. Great way to get XP too, hit the highway (the long stretch beneath the outback festival) on one end, and just floor it to the other end. Avoiding cars along the way will give you insane multipliers.
The BMW M1 is a great car if you want some '80s power. Handles beautifully, if a bit tail happy at times. Convert it to 4WD, however, and it's great.
Staying with BMW, the X5M and X6M are great SUVs. Pretty good in off road jumps as well, and surprisingly nimble.
There are plenty of other cars that are awesome machines, but I can't remember them... Installing the game again now as well, I'll give you a full tour through my garage when it's done. See you online, soon!