Was just thinking about this- I really, really love the final dungeon of Super Mario 3. It's not the hardest or the most graphically impressive. But for whatever weird reason, I really love that dungeon. I like all the alternate routes and this one part after you reach the top of some stairs- there's a little spot to rest and nab a hidden 1-up. It's like the halfway point. And the fight with Bowser was really fun and memorable too. I love pretty much all the final dungeons, but Mario 3 is my fav.