I'm not surprised most people in this thread are picking II, I love pretty much every one in the series but II was one of the first fighters I ever played back at my local arcade and it blew me away: huge damage per hit, fighting with weapons instead of punches (8 year old me was blown away at this) and a really solid fighting system. It had a parry system that predated SFIII by like 3 years, which is an accomplishment in and of itself.
Also to the two people who picked samurai showdown rpg, I challenge you to defend your decision: I played through it recently and found it nigh unplayable. The difficulty is all over the place, the encounter rate is way too high, and the gameplay is pretty much just ripped from final fantasy. I thought the dialogue differences for the different characters was cool, and inputting moves using command motions was actually super fun, but it felt like an honestly kinda bad jrpg, nothing that could compete with the early SS fighting games.