Gohma boss from Legend of Zela: orcarina of time
by far hardest boss i have ever fought on any game

The legendaries are pretty easy to beat for me.

I think the biggest hurdle in the game are those one or two derpy pokemon on a level that just randomly sweep your team.

I remember I was just walking around, minding my own biz when out of nowhere this Spheal just DESTROYS my level 30 Electabuz.

I'm pretty sure it wasn't that much of a higher level than I was but goddamn do those multi-hit attacks pack a punch.

(Soloing)

Smough & Ornstein
or
The 4 Kings

on Dark Souls 1

Bongo bongo was the hardest for me.

Most of the Mega Man X2 (wheel gator) bosses are fuggin hard and I tried to play it directly after beating MMX entirely. Mega Man Zero for the GBA was also very difficult in general.

The fashion witch in triforce heroes..

megaman zx boss forget his name

The dogs in Dog Island

Grinning Colossus in You Have to Burn the Rope

Hinox Bros. (Hinox Mines) and moldorm (fire temple)

Both from tri-force heros.

Currently, for me it's Magus from Chrono Trigger

Omg those guys suck so much.. and the team cant help much if theyre on their first try.

Final two true bosses from Demon's Crest

lol so easy, just wait until Lavos.

Ironically, The boss (prismantus) in the ruins is far easier, even though it is later in the game!

Depression is the hardest boss in everything.