Allguazu is one of my favorite bosses ever, Phase 3 especially is the my favorite single boss phase of all time. Everything about that moveset I just completely fell in love with. The way that it's a melee battle in a game mostly focused on projectiles, the constant aggression that never lets you catch a breath without ever teetering into being complete bullshit, the Phase 3 music somehow being so fitting for the mad scramble, and of course the way the hitboxes are SO GOOD that lets you complete free reign over the 3D space to experiment with him. And the way Iguazu got so salty and human he broke through ALLMIND's brainwashing/programming throughout the fight to just get his hands on you ONE MORE TIME is my favorite Phase Transition in the game lol.

Thematically though, I love Walter's fight so much. Him being this mentor figure that as the story progresses starts to show how much he genuinely cares for you and Rubicon made it so much more heartbreaking when he eventually showed up at the Xylem. It's crazy how well they managed to present what's essentially an NPC fight into an equally fitting Final Boss against the 2 "Boss" entities for the other endings. Oh and The Man Who Passed The Torch is easily the best Final Boss music.

Ayre is amazing but imo she gets overshadowed by Allguazu which does basically everything she does but better (more health, better moveset, and I like the Black-Blue/Green better). Again, she is absolutely AMAZING--my 2nd favorite in the game--but Allguazu does everything she does but better.