Tekken 4. It's not that it was bad, it's just that the improvements were based on glam and bling....nothing substantial.
also a number of the new charcters are wholly unbalanced (which with namco in charge here -is inexcusably bad programming). also the game appears slightly slower than tekken 3 and tag tourney. How on earth is that possible?!
To compound matters, while Tekken appears to be standing still in this incarnation Soul Calibur 2 came out and completely eclipsed it. This just adds insult to injury. Tekken 4 is a decent fighting game, but --no one-- has to settle for decent anymore. If a fighter is not stellar, it will be forgotten.
Bottom line is; times have changed and it's time for tekken to innovate or move out of the way.