My favorite battle was the Final Ghirahim Battle in the Sealed Grounds. I really enjoyed the setting, and it was fun knocking Ghirahim off the platform and preforming a finishing move from such a height. That was really cool. His second phase was pretty tricky and engaging. You had to time your shield or spin attacks just right to get him to drop his guard, and I also really liked that you could have the classic "energy ball tennis" match with his Skyward Strikes. That was seriously awesome.
That last phase wasn't as good, but it was still pretty heart-racing. You kind of feel like you have to strike quickly or else you'll miss your chance to break his sword, but you really don't have to move so quickly. You can take it slow, and for me, it was a lot harder before figuring that out. After all, he would change the direction he was blocking at a moment's notice, which made things really tricky if you were moving too fast.
I really enjoyed the battle, and the soundtrack fit pretty well with the battle. I really like the soundtrack progression for Ghirahim's battles, too. It's sort of like a canon melody, where the melody progression repeats and gets more vivid every time it repeats, just like the way the Ghirahim battles worked. And each battle's different soundtrack was like the next part of the song that repeats and gets more vivid. I thought that was really neat.
I love all of Ghirahim's boss battles, but the final battle against him is my favorite. Then again, each battle with him is pretty cool in it's own way.