I have Porshe Unleashed, HP2, UG1, and UG2. My favorite is HP2, then Porsche Unleased. The other two I don't ever like or play. Hell, I prefer Midtown Madness 2 to the UG games.
The main weakness in UG IMO is the game focuses too much on pimping the car. I'm not a 12 year old kid who likes blinging. Personally, they should have two modes; poser mode and driving mode. Poser mode would appease the kids, and the driving mode would appeal more to the bigger kids like myself.