Guardian Scout IV - A Major Test of Strength - Breath of the Wild: I know, I know, hear me out. Fighting these guys eventually becomes habit, BUT, the first time, when I was weak and underleveled and with poor weapons and still exploring, it was one of the most thrilling moments in gaming ever. For the one, singular time this boss was challenging, I have to give it a hats off. Same could have been said about my first encounter WITH Stalnox and Lynel.
Sans - Undertale: Not only is this a supremely fun battle to play, but the things he tells you while you play are legitimately great lines.
Mantis Lords: Hollow Knight - For some reason, this one stuck with me. Maybe it's the way they are awaiting you, and then the tension of having to fight three bosses? Either way, this might be my favorite.
Agent Black - Iconoclasts: A profoundly emotional battle, great music, and like most of the game, a blast to play.