I think a game that has gone very much downhill is the Tony Hawk's series, which is quite ironic

(downhill/skateboarding pun). In the days of ps1/2 they were my favourite game and they got better with every instalment up until THUG.
Personally, I very much enjoyed THUG as an experience but looking back, it did ruin the series in a way: it detracted from the original, pure method of skateboarding to a time limit to reach a rigid set of goals and made the game more into a story-orientated adventure game.
Tony Hawks Underground 2 was just plain ridiculous-who wanted to play as a wheelchair-bound kid or a skeleton on a go-kart: it just wasn't what skateboarding is about whatsoever. It was also completely unrealistic.
To be honest, I stopped playing there but judging by reception the following games were just as bland and un-original as THUG2.
One can see why Skate is now the dominant force in the skate-sim market: it took skateboarding back to its roots with innovative controls and realistic situations.
/rant