Re: Post here when you beat a game. I mean it!
I've beaten PGR2 and Juiced and played the hell out of Burnout 3 and Forza to a lesser extent.
Of the those you listed I would suggest that if you have Xbox Live, play PGR2 and stay logged in while you play the single player so you can see how you rank. It's great fun. I put over 100 hours in on PGR2.
Next, I would suggest Burnout 3 and then Juiced. Juiced is quite unique in that the gameplay is strictly arcadey, but the whole calendar system and crew-based racing was quite unique. I actually really liked Juiced.
Definitely stay away from that R: game though. By all accounts its garbage. And as for Forza and GT4, I would say go with GT4, as Forza (to me) just wasn't fun.