Soul Calibur.. or Tekken.

Street Fighter. The fighting style in this game has been used in most fighting games since.

Tekken - The learning curve is immense and the you have to know absolutely everything about every move ( from how many frames it takes, to how many negative frames it puts you in if its blocked etc) The juggle system is amazing too ;)

tekken for me killing button mashers since 2000.

For me it's SSBB. Smash is completely different when you play one on one without obects. It's much more like a traditional fighter. I really like the platforming aspect in the game and the depth of the characters.

Apart from that, I would say Soul Calibur because I love using weapons to fight.

To me, without a doubt it's Tekken Tag. However, I would rank Street Fighter 3 a close second.

I'm really really good at Tekken, lol, so I'm certainly biased.

King of fighters,
5 great games 96-2000, 1 very good game, neowave, 5 decent games, 03, 02,94,95, 11 and only one medicore game 01, so with a 90+ percent good games rate it is a quite impressive series
massive character line ups,
technical gameplay, auto guard for dragon punches instead of priority, plus things like input counters
no fire balls, for the most part,
far more robust movement, running, rolling, 3 types of jumps

more limited air blocking than sf alphas reduces turtling
Excellent character design
great animation (post 96), great music, great looking backgrounds

Soul Calibre for me.
I like the weapons based combat, the graphics, and the fact it's fast. Also they have a nice mix of close range and long rage attacks depending on characters.

Marvel vs Capcom 2

Guilty Gear I just love it and also DOA. I like the fast fighting and I was also good in it =p.