It has to be Smackdown. That is probably the SD game I spent most hours on, and I have played each and every one for hundreds of hours.
I thought roster was awesome, with the addition of the legends and wrestlers such as Goldberg, Lesnar, Nash as well as the usual superstars (HHH, Taker, Cena etc) and personal favourites like Ultimo Dragon, Sean O'Haire, Rhyno, RVD and Lance Storm. It was definitely one of the strongest rosters any WWE game has ever seen.
The first blood matches and elimination chamber made their debut in this incarnation of the franchise, leading the so many great matches between me and my friends, and the CAW mode was the best yet.
I also loved the little features on the game, like when playing as RVD and you did a running attack while holding a chair, he would perform the Van Daminator, and that you could drag people around while riding Undertakers motorbike. Little things like that make the game so much more enjoyable.
There is not much I can criticise this game, I thought it was pretty much flawless at the time, and when I think back now it was a pretty awesome game for the time. Great graphics too.