If we were just talking about the single-player portion of the game, GT5 would probably be dead last on my list. I'm not overly impressed by the AI, there's no save feature for enduro/championship races, b-spec mode is a simplified game of Tamagotchi, and car/cash/exp rewards for high-tier races are awful. In terms of completeness, GT5 feels like it was around 75% at release.
Having online multiplayer, however, makes it first on my list. I stopped playing single-player awhile ago. The last 40-50 hours I've played have been online (racing in 2 leagues) and I've absolutely LOVED every minute of it. For all of it's faults, online mode is the greatest single addition to the GT franchise. It extends the life of the game tremendously. When you've reached your point of "dimishing returns" in single-player (where the time invested no longer meets your expectations), it breathes new life into it.
So, yes, GT5 gets the #1 spot on my list, but for an entirely different reason than past GT games.