For me, it was fighting the final boss on GTAIV (After choosing Revenge as my path).

Hopping from a motorcycle to a plane into the garden of the statue of happiness to shoot down an old friend with an AK47, that's really epic.

Orga Angelus is probaly the hardest boss you'd ever fight (although it's really fun to play online with other people). pic from Phantasy Star Portable 2 ↓↓↓






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hmm i would probably say xaldin or sephiroth from kh2 they were most hardest fun and most epic

Probably the battle with Dark Bowser in Mario and Luigi: Bowser's inside story. The music was epic.

alexanyways : the final boss in mgs4 was briliant,it switched between every great metal gear solid tune from the series,it had a beautiful backdrop,and it was just so fun.
Mind you,the ending was pretty sad =(

Um Cynthia in Pokemon Platinum it was hard to beat her even with action replay

Solaris from Sonic 2006, the actual boss wasnt really that great, it was just the amazing music during the second phase that made it truly epic!!

The most epic boss battle I had was in Metal Gear Solid 3 Snake Eater. Fighting The End on the hardest difficult level. You have to stay hidden in the bush or he'll pop you with a tranquilizer dart, or if you stay in the same spot for too long. He'll eventually find you or sneak up behind you when you are looking through the scope. That was a long grueling battle.

For me it was Donkey Kong for Nintendo.. there was like 3 lvls or 5 lvls of the boss that you had to defeat him and it wasss hardddd but I finalllllyyy beat the game! I think I did it when I was like 13 or 14

The most EPIC of battles I have ever had with any boss was final battle series of bosses at the very heart end of Final Fantasy 7.  I can't say it was overly challenging much because I did stock myself namely knowing what I was about to face, but that did not shift the fact the battle still took forever to beat him!

For me, I think it's a tie between Neo-Exdeath from Final Fantasy 5 and Kefka from Final Fantasy 6. Both were extremely hard... and they looked awesome too. Plus, throughout the game you had built a hatred towards them, so it was all released in the end. I loved them both.

I'd have to say Ganon from Ocarina of Time. Imagine a fighting a big monster who is trying to kill you with his two giant-ass swords in a arena surrounded by fire with the Princess screaming from the sidelines every time you get. Oh, and there's a lightning storm above. Too bad he's easy. :/

I'd have to go with either Lavos from Chrono Trigger or Giygas from Earthbound.

For me honestly, it was probably not so much the boss battle as it was the end game of L.A. Noire. Not what I expected, and it was unexpectedly fast, and dramatic. I love that stuff.

I would say Omega Zero in Megaman ZX. He is a boss from the Megaman Zero series (The 3rd game). You meet him in a corrupted zone as a secret boss. The background from Megaman Zero 3 flickers in a black hole where you fight him originally in the Zero series game. In the Zero Series, you originally fight Omega where Zero has woken up in the very first game, a year later, where Weil tries to deceive Zero that he is nothing but a fake, and Omega, was the real Zero, because he had Zero's original first body. Now, 200 years later after, Omega is still standing there, wanting revenge for beating him 200 years ago, in the very same spot, he awaits for you, even though you are now in Biometal form, he is too. And he even has the same boss music; but it's much more evil this time. Absolutely distorted, plain evil. He plans to really kill you. And how much harder he is absolutely is evidence to that compared to how much of a cake walk he really was in Megaman Zero 3. He WILL kill you in seconds if you are not careful. He also has apparently one of the best introduction lines in video game history. "Ware wa Messhia nari! Hahahaha!" Translated into: I am the Messiah! Hahahaha! (One of the battles I've done can be seen here: http://www.youtube.com/watch?v=-KqP-aM69LI )

Let's see... the most epic boss battle I've played is the final boss from Earthbound. Just the creepy eerie feeling you get from facing Giygas; it's such a contrast to the happy early part of the game. And the way Giygas himself is depicted is amazing too. I highly suggest playing the game or at least searching the fight on Youtube.

I'd have to say my most epicest boss battle was in KH1, with Xemnas (Or what's-his-face's name.)
The dynamics of the battle was pretty epic, there were a lot of stuff going on, and you had to give the game your 100% attention to not die.
It also had three stages, which shocked me, and I nearly died because of the three stages, due to my unpreparedness.

PS- I'm not sure if that's KH1, or KH2. 

Anyway, it was the most epic battle I ever had with a boss. I almost debated about replacing Xemnas' battle to Volviga, LoZ's Ocarina of Time Fire Temple's boss. The fiery battle was pretty epic, too! WHACK A MOLE!