MTZehvor on 11 April 2017
If you had a tough time with Thunderblight, don't touch any games pre 2005 or so. They will wipe the floor with you.
The secret Shadow Jago boss fight from Killer Instinct (2013) rekt me consistently, but secret fighting game bosses have a history of being incredibly difficult, so that's probably not too surprising. Pontiff Sulyvahn (Dark Souls 2) was another one that gave me a ton of trouble. Ultimately, though, I'd probably go with Kaptain K. Rool from DKC2. Only being able to take 2 hits, the required pattern memorization, and the sheer amount of health the boss had; that one kicked my ass.
Some of the boss fights from Devil May Cry 1 and 3 were incredibly difficult as well.
