Well, I had some fun boss battle times:
Julius Belmont (Castlevania: Aria of Sorrow)
In a wonderful twist of events in this game you get to fight the man that was able to destroy Dracula (Well almost but I don´t want to go into details about that...). This was the first time in CV history you could fight a Belmont for real (Soleyuu or how you spell his name and Richter were controled) and this old man uses all posible belmont tricks. This fight was intense and you finnaly got the feeling of how ol´ Drac was feeling in all those years fighting the Belmonts.
Death (Castlevania: Aria of Sorrow)
Another one from Aria. This is by far my favourite fight with Death. In the first phase he is mocking you and just attacks with a normal scythe and some sickles, while he is in the BG laughing. From time to time he uses that light pillar technique. You destroy the Scythe and then the real fight starts. The first time a saw him pulling out that second scythe I was speechless so he was able to get an easy hit on me. LOOK AT IT! It´s a friggin´ double sided scythe. Now you knew he wasn´t messing around. Though it´s not the hardest fight but still it´s a good one. Oh and he doesn´t give up when he dies, his surprise attack killed me the first time
Agni and Rudra (Devila May cry 3)
These two bastards. I hate them yet I love this fight for some reasson. I cannot explain it but I enjoyed the fight. When I found out that one of them is weak to Cerberus I concentrated all my attacks against him. But then when the body dies the other one starts using both swords! He owned me instantly after that...But I couldn´t stop. I could take it to be losing against these two. 10 tries later (oh did I mention that you have to restart the WHOLE MISSION if you don´t have yellow orbs?) I finnaly beat them. It was satisfying!
Geryon (Devil May Cry 3)
The only boss in the game I was able to beat on my first try. But still it is my favourite fight in the game. You don´t see a time manipulating horse everyday! You first fight him on a bridge, when the bridge collapses you get to fight him for real. He is fast and he knows how to use the carriage. He can even pull out a Dio Brando on you by slowing down time and make projectilles appear near you.
Copy X (Megaman Zero)
He may be a copy, but this fight was batter than the fight against the original in X5. He is the only character (besides Zero) who can change elements during a fight. Unlike Zero he changes colors while doing so, which made the first Phase a lot easier (I defeated the first form on my first try, then I thought to myself "I thought he would be stronger..." and then Zero said the exact same thing!). Then he want one winged angel on me and hoooly [parasitic bomb] did I underestimate him. Finding a way to dodge all of his attacks was a challenge, hitting him was a bigger challenge. Seeing him crumbled on the floor made me feel happy
Phantom (Megaman zero 3)
Hellz yeah. I loved his fight in Z1, but here his attacks got upraded and the battle was even harder! Too bad he didn´t have a super move like in Z1...
Phantom Ganon ( Ocarina of Time)
The first fight as adult Link. I simply like this fight more than any other in the game (I expected more from Twinrova and Ganon). Finding the portrait from which he will attack from was thrilling. The ping-pong segments were thrilling and slashing him mindless with my sword was satifying!
Also
Sevenforce from Gungstar Heroes for already mentioned reassons.