Zardnaar
Legend
The basic problem it has (and it's my long-running annoyance with Undermountain, as well as other published megadungeons) is that it doesn't have a low-level area. The beginning area is for level 5 characters.
It's really worth remembering that every Undermountain product only details PART of the dungeon - the maps are a lot bigger and you can fill in your own stuff.
I'm not fond of running "Undermountain Only" adventures, so I'd spice it up with a lot of urban stuff as well, giving extra reasons (generally based on factions) to enter the dungeon. While I haven't run Undermountain that way, I certainly have with Castle Greyhawk (and various versions of the dungeon).
Megadungeons require special handling as a DM and paying attention to what your players want. I love them, but variety is also good.
Cheers!
Waterdeeps easy or a starter set.
I'm thinking newbie campaign they all picked dungeon hack. Not awful is bar to clear.