“It’s probably Matt Mercer’s fault.”
I have a standard introductory adventure in my head that I've used many times, but the actual dungeon part is different every time, with a few common elements that get mixed and matched.
Party is traveling between two points and camps. In the night they hear something crashing through the woods towards them. A wounded dwarf bursts into their campsite, with an arrow in him. He gives them just enough warning to prepare for the two goblins who are chasing him before dying. They find a map on him (or they can track where he and the goblins came from) where they find a cave in a cliff. While exploring the cave the rest of the goblins return, trapping them. An ancient door in the back of the cave (sometimes it's secret, sometimes it's locked, sometimes they need to use the oil they find on the rusted locks) leads to a staircase going down. Below are traps, undead, and some kind of secret back exit.
For first time players it's always a big hit.