7. Gill (Street Fighter 3: Third Strike)
It's so difficult to claim anything from Third Strike to be a worst gaming boss because ever since Daigo's legendary parry, everyone claims expertise in everything Street Fighter. For those who don't play Street Fighter casually though, making it past Gill was a bit of a challenge. His normal attacks caused block damage and when he stunned you, it lasted much longer than normal. The AI would land combos and hit really hard. His super arts were particularly broken. His lesser evil was Meteor Strike, that rained down fire and ice balls that could hit you low if you were blocking high, and if it landed fully could take up a massive chunk of your life bar. The most devastating would be his Seraphic Wing, that covered the entire length of the screen and caused obscene amounts of damage, and chipped off a lot when blocked -- which is the only defense against it. On top of that, his final super art would recover his full life bar if he was defeated while his super meter was full, so if you struggled to whittle his health down at great cost of your own, you had to do it again.