Sunless Citadel is dear to my heart. It's the first module a friend DMd for me, and it's the first module I DMd for my kid (he runs a 4 character party by himself normally when I DM). I love the nature themes, the Gulthias Tree, the crazy Druid (what, a Druid isn't only neutral-neutral anymore... felt weird but I get it.), etc. Meepo, he's a family favorite now,... we have a couple of miniatures of the guy even and reference/joke about him often. And the gnome prisoner joined the party for a while, and my kid still has a soft spot for that guy.
The twig blights, being let loose into the world, really struck home... felt real. We had some side adventures with a treant, shambling mounds, good druid(s) trying to set the area right by ridding it of twigblights, but we know some escaped. Now we're playing Curse of Strahd 5e (years later) and I was partially drawn to it for the reappearance of a Gulthias tree and the twig blights (our old enemies that we...er the party couldn't stop!).
In regards to others' comments... you folks reminding me of some good or perhaps nerve racking times - the kid almost got in big trouble from those rats on the rope drop, yep, but we managed it. The white wyrmling could have been scary... I bet the party was above level 1 by then, but the way I recall reading it... it was more a "pet" than not, so sort of tame (likely myself the DM took it easy). Maybe the pet aspect came into play more because the party definitely was more on the kobolds' side than the goblins. Well, I'm off to read the rest of the comments, good stuff.