Competitively? II
No other game captivated me as much as this one. It is the sole reason I play FGs competitively anymore, let alone have any interest in the scene. A most worthy sequel to its predecessor(IE: Project Soul - This is how to do sequels)
Creatively? (Art direction, story wise? etc) SC1
The single best arcade-console port I've ever seen. Had everything you could've wanted in a game at that time and more. Fantastic art direction, engrossing tales of fiery souls and clashes of steel, adventuring and all that awesome jazz. The series peaked here, I think.
Honorable mention to Soul Edge, which got everything started.
III would have been great if it been produced with half as much polish as T5, but alas, good art direction and music alone doth not a good game make. IV was okay. V, we shall see. Too young yet to pass judgment, but it already loses in the story department(that's what happens when you cater to a wider audience, I guess, still sucks).