for me it's...

Brayflox's Longstop HM: It's a quick little dungeon with some fun fights and I have to say The first boss (Illuminati Commander) is my favorite. The Bomberman bombs make the fight fun and funny. I'm a little sad this has become THE place to Speed run and get seals, but when you're with a group that doesn't feel everything is a race, it's a lot of fun.

Lost City of Amdapor: I don't run it much due to how slow it is (I'm not complaining, it's a time issue on my end, making LC hard to fit in). but I like most people find the atmosphere and the Diablos fight is the best bit, neat mechanics that on first blush are confusing and a little frustrating, but once you understand it, it's a neat trick and pretty fun

And Believe it or not.... yes Copperbell Mines (NM) I love this dungeon because it is the first one that really asks you to use your head and introduces "Boss Mechanics" How many people here on their very first run through tried to DPS down the blob (no one can hurt "THE BLOB"!) and thought"OMG! KILL THE BOMBS" before figuring out you needed them to kill the blob. and while Satasha introduced Add mitigation, Copperbell teaches the Timed Add concept and actually forces the teaching of, you gotta kill the adds. which made for an early dungeon being actually kinda fun.