I absolutely loved all the
PH bosses. They're not hard (not even the final form of Bellum, contrary to popular belief. There's an obvious pattern to his attacks and his reactions when you attack him. It's easy), but they're so incredibly fun and innovative.
The first boss was lame, a little bit after the style of Agahnimh in ALttP, but all the rest of the bosses were so awesome. I love the different angles and viewpoints in each of them. Like the invisible crab-guy, where the top screen shows you his view and you have to watch and shoot him quickly. Or the giant stone soldier Eox, where you have to fling yourself in the air and pound away at his red dots. Or, the two-headed dragon, where you had to diagonally redirect his shots to the different heads. Or the first stage of Bellum, with the three-story 3D thing where you have to shoot his eyes from across the room. The final two forms of Bellum were lame, however. I wish you could play the first form again, instead of just stupid Linebeck-Bellum. I've created a whole new save file where all I do is fight the bosses over and over. I'll deliberately die before beating a boss just so I can play him again.
I could go on ranting about how much I love the boss battles all day, but I'm sure you're tired of hearing about them already, so I'll just leave it at that.