There are certainly many memorable bosses, plus ones which I found were... average. But my personal faves (spoilers incoming?):
Hewdraw - if the multiple dialog choices wasn't awesome enough (head order is how you do it), then running on Hewdraw's back in the end of the sky battle must certainly be awesome.
The end of the land battle boss fight is not as good (plus I do pretty averagely in it above 5.0...)
Thanatos - the pot transformation is the only really annoying part of the battle, the rest is comical especially with dialogue.
Phosphora - admit it. She's hot. And a complete combat pragmatist in the intro video.
Chariot Master - the unexpected battle on horseback is just a marvel to experience. Plus it's not too tough a battle either.
Amazon Pandora - aside from the fanservice, kicking Calamity Goddess butt (literal or not) as DP is awesome. Especially his snarkiness rubbing against Viridi and Palutena's commentary.
Hades' Heart - It's Bomberman in an alternate Hades-like dimension! Oh and it hurts like hell.
Pit's moments of madness make this funnier.
Magnus and Gaol - the tag team against you makes it quite a challenge but being situation aware is at least half of what I enjoyed about this battle.
Great Sacred Treasure - well duh, fighting a giant mecha with instant death attack and the like.
Hewdraw - if the multiple dialog choices wasn't awesome enough (head order is how you do it), then running on Hewdraw's back in the end of the sky battle must certainly be awesome.
Thanatos - the pot transformation is the only really annoying part of the battle, the rest is comical especially with dialogue.
Phosphora - admit it. She's hot. And a complete combat pragmatist in the intro video.
Chariot Master - the unexpected battle on horseback is just a marvel to experience. Plus it's not too tough a battle either.
Amazon Pandora - aside from the fanservice, kicking Calamity Goddess butt (literal or not) as DP is awesome. Especially his snarkiness rubbing against Viridi and Palutena's commentary.
Hades' Heart - It's Bomberman in an alternate Hades-like dimension! Oh and it hurts like hell.
Magnus and Gaol - the tag team against you makes it quite a challenge but being situation aware is at least half of what I enjoyed about this battle.
Great Sacred Treasure - well duh, fighting a giant mecha with instant death attack and the like.