I'm an huge fan of Tekken since 2. Even now I recognise the cleverness of Tekken 4 (the bridge game that allowed them more freedom with their style). My favourite of the series is easily Tekken 5 (or the later DR version). While Tekken 3 was great for it's time, Tekken 5 is in my view the pinnacle of the series.
Sadly things have been lost with Tekken 6 and Tag 2. 6 is good, however I feel very unbalanced. The bound and juggling mechanic was a bad step for the series, and they made Lars for this mechanic, giving him an advantage over other characters, in my opinion, him and other characters that have easy to perform/confusing 'lifts' gain an advantage. I much prefer to play the traditional way, where you just try and beat the crap out of the other person, even if it means kicking them when they are down or trying to get up. Tag 2 is a spammers heaven, a mess of a game. I'm on the ground, get kicked and suddenly bounce up randomly and they are doing a new tag combo? I mean what the hell.
On to other series, I've not got many from this generation. I don't feel they are a good and varied as they used to be. Not got into Soul Calibur or Dead or Alive. Late 90s was the fighters time.
Star Gladiator is a fantastic game, wish they made more and carried on the series (Not played the DC sequal).
X-men Mutant Academy 2 - a great game, far better than the first, I've played SF EX2 Plus and it's better than that I'd say.
Marvel Super Heroes - loved this one of the Capcom 2D fighters.