Cragmaw Cave from Lost Mine of Phandelver. An almost perfect starter dungeon for beginning players and characters, with a surprising amount of options that leads to some nailbiter tactical combats.
Cragmaw Castle from Lost Mine of Phandelver. Like Cragmaw Cave, this seemingly simple map has surprising depth. I recently ran a 10th level group against a group of hobgoblins on this map. While the PCs weren't ever really threatened, the fight was unexpectedly thrilling.
Wyrmheart Mine from Tomb of Annihilation. This one I reskinned as the lair of a young blue dragon and his kobold minions. The players had to fight their way all the way from the bottom to the top. It was epic.
Wyrmheart Mine from Tomb was awesome, I forgot about that one. I had them find Tinder deader than hell on the bottom level, ripped up by an even bigger dragon I named Magmus. The party had Artus Cimber with them and that damn Ring of Winter, and I knew Tinder wouldn't cut it, not even with the kobold minions. But it worked; Artus and another PC died fighting Magmus and the Ring encased Artus in a growing block of ice and was removed from the campaign in a super dramatic fashion.