Probably Dark Souls 1. I love how much the Soulsborne series builds up its bosses with lore and environmental storytelling. That's just as important as the actual gameplay challenge itself. Dark Souls 1 is probably the best because of varied the bosses are, and the number of epic confrontations that just make it feel like you're doing something really important. Ornstein and Smough, Artorias, Gaping dragon, Kalameet, Priscilla. All of those are epic confrontations.