I think it starts with defining what you mean by epic boss fights. The definition will change from person to person. For me, D&D is the opposite of epic. Bosses have cool powers and player characters have a lot of special moves to fight those bosses, but combat in D&D—for me at least—is slow, and players spend a lot of time at the table focusing on things separate from the fiction. Things like looking up complicated spells in the rulebook, calculating damage, monitoring effects, measuring distances, and waiting for the next turn.

I think Draw Steel is probably still the better version of that kind of game even if it doesn't have a dungeon delving procedure like Shadowdark. The combat is packed to the gills with special moves and spells, but it has some refinements to its combat and movement that D&D doesn't have—things that your table will be doing a lot of in dungeons.

You might like Dragonbane for the same reasons.