I've run Sunless Citadel, originally published for 3rd Edition and most recently printed in the Tales from the Yawning Portal book, a few times and thought I'd give my thoughts on running it in either Pathfinder or D&D. In my experience, it's more or less a perfect adventure for almost any purpose. It's flexible, at least within the confines of being a dungeon crawl, with lots of controls for making it a short or a long campaign, and a diplomatic or hack-and-slash game.
Potential players: This post includes minor spoilers, but only if you plan to play it and you want everything to be a total surprise. I'm not discussing any plot points, but I mention mechanical aspects that may reveal a thing or two about the map and general setting.
The module as printed in TFYP is just 21 pages, but don't let that fool you. It took a group of 3 players 8 weeks, playing 2 hours a week, to complete the adventure, and even then there were rooms left unexplored. There are over 50 locations in the adventure, and that's not including the nearby town.
And the initial map you see in the book is just the first level. There's a lower level to the dungeon that's not evident until three-quarters of the way through. And the lower level is not small.
Fortunately, the plot of this module is both simple and flexible. As Dungeon Master, it's easy to keep the story focused or to allow the players to draw it out as long as they want. Here are the "control knobs" I've used:
Long: Tell the players about the goblin and apple plot line and the missing adventurers plot line.
Short: Use just the goblin and apple plot. As they delve into the dungeon, they'll learn more about the origin of the apple, and come to understand their ultimate goal. The missing adventurers are relegated to unexplained backstory elements.
Long: Leave it up to the players.
Short: Plant a secret door somewhere that leads to the boss fight. Alternately, there's nothing saying the boss can't be mobile. Bring the fight to the players.
There are two warring factions in Sunless Citadel, and your players can either choose sides or attempt to bridge the gap between them and come to a peaceful resolution.
I feel like the adventure assumes players are going to choose sides, but arguably there's no right choice, at least not within the bounds of alignment. Do you want to help the kobolds nurturing an evil dragon? Or do you want to help the goblins assisting an evil magic user? Or do you want to help them both? Or do you just want to kill them all?
As Dungeon Master, you can play up the evil aspect of either faction, or both, or neither. If your players are leaning toward diplomacy, both factions can easily be seen as victims of an external force. If your players just want combat, it's just as easy to make one faction vile, evil, hateful, and aggressive.
Player characters are meant to advance from 1st to 3rd level during this module. I don't know whether it's because I hand out XP unilaterally or just because my players tend to be completionists, but my players usually reach 5th level by the end. That makes them just a little more powerful than they probably ought to be for the final battle, so I turn the tree in the grove into an evil Treant (such as a Weeping Treant from Kobold Press's Tome of Beasts.)
To me, this adventure is nearly everything D&D ought to be. It's a dungeon delve, but the environment has a rich history to it that encourages exploration. There's a story happening in the location that's not all necessarily about the main plot, but it all contributes to the plot. There's choice between diplomacy and combat. There are a few minor moral quandaries but it's a reasonable amount that ought to encourage player discussion without breaking the game.
It's not a one session adventure, unless you adjust the pacing, but it's a great way to start D&D.
Cover image copyright by Wizards of the Coast, used under the fan content policy.