The last one was the best. I thought it was the one with the most quality. I also thought it was really fun. When I hit him off the ledge 3 times, it felt so good. And it was fun hitting him with my sword. And the big rock weapon he had was AMAZING.
I liked the last one for one reason only. When you knock him down a level and you have to do the fatal blow thing, I back up as much as I can and boom. Link flies across the stage and slams the Master Sword into Ghirahim's chest. It is so awesome to do that.
The final one was my favorite. The first two I found annoying. Even after several playthrough, I'm still not sure how to swing the sword so he doesn't grab it. Luckily the third and final fight adds new elements and makes it interesting.
I like the final battle the most, particularly knocking off of the platforms but didn't like having to do away with his sword a little later since it healed itself. I didn't take much damage and i never died but it did take me a long time to eventually break his sword, but i really liked the way he looked and the sounds his body made when you hit it, very delicious. =]
My least favourite was the fire sanctuary though, the setting didn't fit and he didn't have much business being there. One of SS's weak points were its bosses and I really disliked having to fight the Imprisoned and Debbie three times throughout the game but at least Debbie stepped it up for the final fight.
The third fight is too epic to deny it as being my favourite. The constant use of the fatal blow in the first form has to be the highlight. Additionally, the only appearance of dead man's volley is in this fight, and it's optional. A definite plus.
I liked the last one in the sealed grounds. He really loses control here, and it's both fun and a little freaky watching him turn form sadistic to throwing a tantrum, calling you a "BRAT!!!" I liked the Skyview one 2nd best because of the intro cutscene. I think that the Fire Sanctuary fight is the worst one for several reasons (see http://zeldadungeon.net/forum/f54/volvagia-fire-sanctuary-boss-32694.html for details).
The first two Ghirahim battles were copypastes as far as the strategy needed to beat him, and the third wasn't all that interesting either. I give the flag to Ghirahim III, because beating the tar out of his giant sword felt cool, oddly enough.