What do you think is the best developed thing in the game? It can be anything like a skill, perk tree, NPC, city, town, activity, questline, etc and what do you feel is the most undercooked part of the game?

The world design in general is very well developed. So are the dungeons, sometimes still repetitive but a huge improvement over Oblivion's copy paste dungeons.

Least developed is the A.I. Especially when you have level 100 sneak it basically breaks the A.I.

The world and dungeons. Indeed. And the character design, it is way better than Morrowind′s old ones and that the blasphemy of Oblivion.

Less developed? The guilds and civil war.

The most developed: the opening from beginning to whiterun
The least developed: guilds and questlines

Why just the beginning to Whiterun?

Most: the game world. Intricately detailed and filled to the brim with stories, Easter Eggs and other things to find.
Least: the College of Winterhold, Companions and Civil War questlines.

It feels more staged and eventful, like they wanted to hook you into the main quest.

Most detailed are the combat and early gratification. Least detailed are meaningful npc interaction and recognition.

Best developed: The world overall, even if it does start to feel a wee bit small after a while.

Least developed: The stories/questlines overall (Particularly the Dragon Cult) and magic.