If you got more than one, definitely post it. My two favorite had to be Hades and Hercules. Zeus was really disappointing. I didn't know I was playing Street Fighter. Lol. Hermes was also pretty disappointing.

Hades and Chronos. 

" @Akeldama: Chronos was good, but I don't know. When you repeatedly smash Hercules's face in, you can't help but say "Wow.... how freakin' awesome." "

Hades...maybe

I don't know about you but I never stopped pressing the circle button to smash Zeus's face in, I'm still pressing it now.
 
EDIT: Still going.

Chronos

Hades, definitely.

Hades was the best boss battle in terms of mechanics. While Poseidon on top of Gaia had the largest wow factor.  
 
Hermes was fucking annoying. And the first part of the Zeus battle was terrible, and by the time I finally beat it and got to the next part of the battle I felt like I forgot how to play. Which then took me a while and left me with a sour taste in my mouth.

Hydra fight hands down.  Still my favourite boss battle of the entire series.  The sound of the roar, the blurring of the screen.  It was amazing when I first played it and I still find it amazing today.

Hades without a doubt, fucking tug of war, man! Followed by Poseidon and then Cronos.  
 
Zeus was definitely underwhelming, and I found Hercules pretty lame, too.

Cronos followed by Hades. Enjoyed all of them but those two stood out.

favorite boss battle? Hades Cerberus Breeder with Satyrs in Titan mode.

Poseidon was one of the best starts to a game ever.  I also really enjoyed the Zeus fight in front of Pandora's box-although I died at lease 20 time in that sequence, I never really got mad or annoyed.

Zeus' 2D fight. Mostly because it was the only one that didn't bore me to death.

Poseidon. 
Hermes was the worst.

I like Chronos
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Hercules and Chronos :D

Chronos of course.  They need to make a game with just titan killing.

Based of Titan Mode which imo is the way to play this game for a more than just button-mashing experience. 
 
1.  Poseidon, after completing the game on normal and wasn't sure what to expect in Titan mode - I was raped a good amount of times just on the first segement of this boss. A great boss in scale and movements.  
 2.. Hades - Thank god I did this on normal first to get a feel of the mechanics, otherwise I could've seen myself spending hours on this encounter on Titan-Mode. Nice use of mechanics in this fight as well. 
 3.. Cronos - This is more up here due to the scale of the fight. The scale is what makes this fight. Nothing hard or challenging, but the visuals and scope are what this memorable. 
4. The Scorpion boss - honestly fuck this boss. The randomness of the big ass tail swipe hit that does huge amount of damage was retarded.  While you can throw the scropion adds for infinite health, it was very tedious. Besides those parts, this was a cool boss that could've been higher. 
5. Hercules - Decent fight with a simple mechanic, nice scope and presentation but pretty standard fare compared to others imo.  
 6. Zeus - The 2D fight was ok, pretty easy and standard fare - the final fight was pretty easy. Sucks it had to end this way, but the early and higher ranked fights are all incredible.
7. Helios - A boss fight where its the pre-fighting is the actual event. Not bad but again compared to others, nothing great.  
 8. Hermes - Nice fast paced pre-boss action, but the actual boss fight was underwhelming only to have it servicable after the death scene

I'd say Hades or Hecules!

Beating Kevin Sorbro's face in, though  Harry Hammlin is still my all time favorite GOW boss fight.

Hermes was definitely the worst, although I still enjoyed it. Especially the pre-chase and the blood trail. My favourite was probably Hades, Cronos a close second.

Such a brutal game, but the remorselessness while beating in Hercules' face was shocking and memorable.
 
Not sure it matches the overall battle with Cronos, though. The minute he tossed Kratos down his gullet, I cringed at the thought of how he might escape. My guess? Back door.

I'll go with Cronos on this one as well. What scale!

The Poseidon fight, the Hades fight, and the Chronos fight were pretty freaking amazing.  
However, the Colossus of Rhodes is still the best in the series in my opinion.

I'm going with either Hermes and Hercules.