Omega Weapon from FF8 without Holywar/Limitbreak strategies
Omega Mk.12 from FFXII.
Level 50 Ruby Weapon Solo bluemage mode in FF7.
Emerald Weapon Solo in FF7 (Red XIII FTW)
Iselia Queen from SO2 on Universe Mode (hardest fight in any game I've played IMO, and funnest).
All of the above fights have a level of difficulty that can be adjusted based on the strategy u employ or restrictions you choose to put on yourself, except Iselia queen which requires completely maxed out game file plus tactical awareness and strategy with no scripted mechanics at all.
If we are talking about an MMO, an example IMO of a great boss fight is Draigoch's Lair from LOTRO. 24 man single boss raid that requires positioning, movement, OHKOs (not for everyone), great music, and multiple setups.