It really, really depends what you're looking for.
Classic NFS games were amazing arcade fun, but not really realistic at all. Running from the cops, etc, the best for that was Most Wanted in recent years, but the best overall NFS arcade game was Hot Pursuit 2.
As for NFS customization/freeroam. Underground 2. No doubt about it. Talk about an amazing game. Carbon was passable, but nowhere near as good, and the first Underground just hadn't quite got things yet. Prostreet was mostly a failure besides in graphics.
For simulation? Forza 2. There's really no comparison, it just blows Grand Turismo out of the water. More cars, more parts, real tracks, and holy hell, realistic driving. Everything in it is amazing.
For party? It's a mashup between Crash Bash and Mario Kart 64 for me. Both amazing party games regardless of age.
And finally, for that elusive hybrid game, half simulation, half arcade? GRID has you covered. It's a lot of fun, the graphics are stunning, and it's not too much of a simulation. Project Gotham 3 takes the runner up in this category.