Let's see... going by the thread title for best ever... I think I'd have to say Soul Calibur 3. Though there are a few things they did better in Soul Calibur for the Dreamcast, overall 3 was more fun and better balanced (as long as no one decided to specialize in Mitsurugi or Voldo).
In the past three years, though (protip for my fellow posters: read the poll heading. That's why older games aren't listed)... I'll admit I haven't played many. Soul Calibur 4 finally took the series off the deep end, the mechanical balance and intuitive controls are just gone... Super Smash Bros Brawl is a good drunken party game, but not technically complex enough to really qualify for a place on the list...
So I guess I'll have to go with another game that may or may not qualify: Dissidia Final Fantasy. Terrible storytelling, but oddly engaging gameplay.