Hmm, 06 or 07. Both games are pretty much the same. 07 may be a bit faster paced as a game, but they're basically the same. After 07 I feel like they gradually fell off the licensing-train and had a gradual decrease in licensed drivers. The 09 game didn't even have car-badges.
All games does have some car badge/shape trickery and some missing drivers. 2008 isn't any better than 07. It has new hud, and the Car of Tomorrow. But I never got properly into it. They also removed the team-control aspect of it. Yes, it wasn't realistic that you could jump between cars in your team during the race, but it really did give a more "team owner" feel.
Basically, it comes down to which drivers you feel you "must" have in a game, these are missing:
NASCAR 06: Dave Blaney, Robby Gordon, Bobby Hamilton jr, Bill Elliot and Mike Wallace.
NASCAR 07: Carl Edwards, Ken Schrader and Clint Bowyer.
For me 07 is "worse" in that respect, due to Carl Edwards. Clint Bowyer did appear in NASCAR 07, but only in the Nationwide/Busch series. There is obviously loads of drivers missing in the lower series. Basically every driver with #100 or higher is fake.
NASCAR 06 also had more alternate car liveries IIRC, which made the races feel a bit different when the AI's suddenly had different paint schemes.
They all look pretty bad though. They didn't look great graphically back then, and they certainly haven't aged well either. I still play some of them now and then. I like the career-part 
You certainly weren’t joking about the bolded bit! I was passing CeX today and managed to find NASCAR 07 on the PS2 and 08 on the PS3, so bought both. I tried 08 first and think it has potential, then put 07 on and couldn’t believe how bad the graphics were, it’s like a PS1 game. But actually, I suppose like actual PS1 games, you start to see past that quite quickly and it’s certainly got a nice feel to it. I’m on half term this week so will have a proper go on both when I can, always been keeping an eye out for these after 06 wouldn’t work in my PS2.
