Definitely PGR 2.
PGR 3 and 4 look nice and pretty, but that's about it. This will sound wierd, but the graphics are almost too much... the cars look like shiny plastic toys.
PGR 2 may not be the best looking game (it does look pretty good in HD though) but the cars look "better" and have a more realistic feel to them.
It's the substance and gameplay that really make PGR 2 stand out from the rest. It has some of the best handling physics out there to date. The online collision detection is almost perfect, none of that tapping your opponent and spinning them out. In PGR 2 you can actually bump your opponent and they won't go flying off into the wall. It also has a much better overall Kudos system, not to mention the leaderboard design, layout and implimentation put PGR 3 and 4 to shame. You also have the Ghost Challenges (weekly, monthly, and all time). PGR 2 has something for everyone, Kudo whores, time attack junkies, Cat and Mouse, Last Man, Ladder Matches... it has it all.
PGR 2 Rank=>60: 25th overall, 100 Million Kudos