Moto GP 07 is the best of the lot, because you have other types of bikes with all the upgrade options on them. It started going downhill when Moto GP 08 came out
For me after 07 they started to get/feel very arcadey!
Like Macca said, the SBK games are really good, I've had all titles released and whilst there's never a great difference in them but they have always made slight improvements, would definately recommend SBK Generations as a rental, or if you wanted to buy
it's £21.85 on Shopto ( It does say though last few in stock )