this isn't really a boss battles, 

Omega Weapon FFVIII, Limit break spam fest. I loved that!
And, king bomb omb (or whatever the hell the bomb on top of the mountain on mario 64 was)
Paz from peacewalker.

Notable bosses that didn't make it to my top 5:


Goth - The Legend of Zelda: Majora's Mask

Biolizard - Sonic Adventure 2 (Battle)

Big Bob-Omb - Super Mario 64

Nichole - Dead Space 2

Pious Augustus - Eternal Darkness: Sanity's Requiem

  Gattuso ??

Most memorable - Capra demon from Dark Souls springs to mind, not sure why, I got trolled hard by that boss, even though on the killing attempt I barely lost any life.

Favourite - the human reaper from ME2.

God of War III = Poseidon, Hades, Hercules, Hermes (including the epic chase), Cronos, Skorpius, Zeus.

God of War II= Colossus, Theseus, Barbarian King, Euryale, Perseus, Kraken, Sisters of Fate, Zeus.

God of War: GOS= Scylla, Callisto, Erinys, Piraeus Lion, Deimos, Thanatos.

God of War= Hydra, Pandora's Guardian, Ares.

God of War: COO= Persian King, Basilisk, Charon, Persephone.

Luca Blight from Suikoden 2. SPOILERS AHEAD...

Most boss battles in RPGs take just the one go. One team, one fight, done.

Luca Blight gets ambushed by an army, shot with arrows and then takes on 3 teams of your best warriors. All 3 teams can have up to 6 characters in them. None of these teams kills him. He wanders off into the forest, laughing... There's a trap - He gets shot with a bunch more arrows from your army. Your main character takes him on in 1 on 1 combat. You win, and he dies laughing.

His final words - "It took hundreds to kill me but I killed humans by the thousands. I am sublime!!! I am the true face of evil!!!!"

The battle theme is quite awesome, too - http://www.youtube.com/watch?v=MX8fKtjpr_E

Chrono Trigger:
- Lavos: Just epic, and that music...

FFVI:
- Kefka: Heck yeah! Revenge!!!

Paper Mario: TTYD:
- Shadow Sirens 2: I loved it, it was the long awaited final battle for Vivian.

Mother 3:
- Pokey: At last, we can destroy that spoiled brat.
- Masked Man 2: Very sad, and interesting battle.

Zelda: TP:
- Final Battle: Epic and interesting.
- Desert Battle: One of the most creative battles ever.

Mario RPG:
- Culex: A nice change and a very epic setting.

DKC:
- King K. Rool: A very surprising battle, and fantastic music.

These are some I remembered right now.

Yiazmat (final hunt) in Final Fantasy XII, It took me 18 hours 20 mins (to beat him alone) but the last few hours when he is in critical at double level (Level 150 ish I believe) taking only 2/3rds damage and killing a (lev 99 with ultimate equipment) party member almost every hit is just epic, such an intense fight.

Plus add the hardest monster from any given monster hunter game in here, their toughest is always a blast regardless of the monster hunter game/version. But that basically IS why anyone plays the game

Speaking of which I need to buy a Japanese 3ds so I can get M.H 3G....

2000cc said: Yiazmat (final hunt) in Final Fantasy XII, It took me 18 hours 20 mins (to beat him alone) but the last few hours when he is in critical at double level (Level 150 ish I believe) taking only 2/3rds damage and killing a (lev 99 with ultimate equipment) party member almost every hit is just epic, such an intense fight. Plus add the hardest monster from any given monster hunter game in here, their toughest is always a blast regardless of the monster hunter game/version. But that basically IS why anyone plays the game Speaking of which I need to buy a Japanese 3ds so I can get M.H 3G....

Yup, that's a very good idea. Apparently it offers a lot of bang for the buck. 93/100 from NGamer!

5. Spider alien creature - Resistance: FOM
4. Giant whale fish - RE4
3. Anton Gerdeux - Syphon Filter 1
2. Laughing Octopus - MGS4
1. Alden on the bridge - inFAMOUS 1

I'm sure I'm missing some major ones. Haven't played MGS2 and 3 yet, so I'm sure that would replace some here. Good topic!