Fighting Ares in God of War was amazing for its sheer epicness, even though it was hard. Also, fighting Kronos the Titan in God of War III was incredible, the whole sequence was just badass. Finally, fighting King K. Rool from Donkey Kong 64 was fun in that you had to utilize all of the Kongs. I am sure there are more, but I just can't remember right now.
From the top of my head I can say Queen Remedi & when she transforms intoLi-Grim in Final Fantasy Tactics Advance for the GBA <3
There is also one in the Valkryie Profile 2: Silmeria that's i really enjoyed cause it was a bitch to beat him, but I forgot his name D:
Or Red in Pokemon All pokemon battles are FUN!
because I love final fantasy 7 SO MUCH, the first time fighting Sephiroth really pumped me up.
but I think my favorite boss fight is with Emerald Weapon. it took me an hour and 30 minutes to finally beat Emerald Weapon. i really liked that fight because it used a lot of strategy in the game. and a lot of times i would be 45 minutes into the fight and lose. XD i would get so mad.
The battle against Yunalesca in FFX hands down. There was so much emotion in that battle and the events leading up to it were marvelous. It was also one of those battles in which you hear about her throughout the game and have the image built up and then when you get there and find out everyone is casting off their belief and fight her? Yeah, it was awesome.
I really enjoyed a lot of the Esper and boss battles in FFXII too. The battle against Gilgamesh was probably my favorite. I just really like that battle system xP.
Well God of War and God of War II were originally released for the PS2, so maybe you could pick those up more cheaply? And they were packaged on a single disc and remastered as the God of War Collection for the PS3. Just thought I would give you some options