I will leave my top 5 of bosses that have cost me the most to beat.
1. I mostly only play old games so I won't mention new games.
2. I will not mention Mike Tyson or megaman bosses because they are highly mentioned bosses.
Well here I start...
5. Jashin (Ninja gaiden) put him in 5th position because besides being difficult since his projectiles were a bit fast if you lost you had to start from 6-1 again. God
4. Sauros (Shadow Dancer) Again the ninja games, and this boss is not far behind. Worst of all was his unpredictable enemies and what else should you trust with luck. Extremely tough, but I was able to defeat him.
3. Fortunia (Pokémon) Probably the most difficult pokémon battle and even more difficult of the battle frontier. (Destra or brandon could fight him) She had a really brutal team, almost always her pokémons were made to lower yours, or even a random attack that weakened you a game over. And yes, you had to start the whole long challenge from 0.
2. Kaiser Greedy (Ristar) For obvious reasons this boss has given me headaches, not only was he fast, he had a difficult pattern, he also cheated a lot in battle. You not only require a lot of memory, but also good reflexes, good luck with it.
1. Metal Eggman (Sonic 2) I put it here because not only was it difficult, the slightest mistake made you start from 0, it was difficult to hit and especially a long series of two battles with several hits and without being able to take damage at all the fight. I have suffered a lot here.