Objectively, I'd say the worst fighting game I own is Guilty Gear 1. Yeah it's the start of the series, and it is fun to revisit once in a while, but no way in hell would I say it's a well made fighting game.
Subjectively, I hate Mortal Kombat and Mortal Kombat II. They were definitely more about the gory spectacle than being good fighting games, and this was still the early 90's when everyone was making fighting games to cash in on Street Fighter II.