The king of boss fights for me has got to be the Touhou series. They do however only have 7 boss fights per game, so hard to say they have a 'ton' of them, and the quality does vary. Still, notable mention to two games: Touhou 11 Subterranean Animism, for having an absolutely stellar lineup of all memorable bosses, and Touhou 15 Legacy of Lunatic Kingdom, for still managing to wow with bosses when you think you've already seen it all in these games.

Outside of Touhou - and this is prolly gonna be a *huge* surprise to people who know my tastes - I'm gonna give it to Ys: The Oath in Felghana. The boss fights are just so much fun in that game, with a lot of standouts. Honorable mention to Ys Origin which definitely has highly memorable bosses as well, it's just barely beneath Felghana for me.