to me the last good one was 4,after that the series declined since stopped really improving the mechanics of the game and just started adding gimicks. Underground introduced walking but it didn't really add any serious depth to the core skating experience and it let to pointless add-ons like driving and such. It's also here that the series tried to add flavor by having a storyline (and a boring one at that)that was filled with supposed "skater' attitude, lingo etc. In Thug 2 they ramped this up to eleven(or just practically made it jackass with skateboards) but even then the games had an a certain charm to then. In this generation of consoles and they took that out and it looked like they were gonna focus on the series, but even then just did nothing but kept adding pointless additions to the series, making it lack charm and innovation. Couple that with what seemed decreasing amount of effort into series (anyone who has played proving grounds can see that point, what with so much errors in collision detection and other glitches that it seemed the people at neversoft didn't really care anymore) made it easy to get dethroned by skate which actually brought some innovation to the genre.