TW3, easily so. The writing is great, as is the voice-acting, TES games are notorious for their poor writing and VA.
Quests are on another level; Skyrim quests are basically run to the quest marker, slay everyone in the dungeon, go back and get ten gold pieces. Motivations, characters, and objectives are all stylized and shallow. Almost all quests are variations upon the same thing, over and over. One plotline that could have been interesting with more depth and breadth is the civil war. Not only does it play out in a ridiculous and tedious manner; the choice you make has almost zero impact on the game itself and its world. Choices don't matter, which makes the choice worthless to begin with.
Skyrim's combat is completely without weight and feel, it's a rock 'em sock 'em simulator with poor sound design on top. Enemies have very little variation, and their AI is shoddy at best, hilarious at worst. There's also no strategy to the fights; approach and whack until dead, or sneak and loose arrows until dead (often a single arrow).
One area where Skyrim shines is atmosphere, and the brilliant music does it a service as well. The problem is the world itself, it seems anemic and dulled down, characters are flat caricatures of people and no one shows signs of proper intelligence. Convictions are all childish rage and simple need-to circumstances to drive the plot. It's just poor design. Especially cities showcase this issue, characters are either sitting around eating and drinking all day or standing/walking aimlessly around the streets, tossing nonsense at you as you pass them by.
With a bunch of mods, Skyrim is still a great game. But the changes made to appeal to a broader audience are the very same that alienated it from fans of the older TES games, and indeed the RPG genre as a whole. For me, there's no contest here, and while they are different types of games, one could argue that the essence of the TES series is a direct competitor to TW3 and its core design. To say that they're too different for comparison is too easy, in my humble opinion. It's not Trackmania vs Gran Turismo.
I've played a lot of Skyrim, and a lot of TW3, the key difference is that my two playthroughs of TW3 were one continuous stream of being engrossed and captivated by the world and writing and characters. Skyrim, on the other hand, I've found myself making at least 30-40 characters, and I've only ever finished the main plot twice, it's just not tempting and captivating. There's not a single character I ever cared about at all. I always find myself bored after a given number of hours, when I realize that my choice of "build" makes no difference either. Which race I pick has no proper bearing on the gameplay, only a few snide comments coming my way from time to time, and skills and weaponry and/or spells and magic is built such that everyone can always get equally good at everything. 7-foot orcs can pickpocket and sneak like 5-foot wood elves (and the wood elf can smash with a warhammer in the exact same fashion as the orc), Argonians get the same prices in shops and taverns as Nords and Imperials, everyone can use magic, everyone can dual-wield and wear the heaviest armor, and even forge this themselves. Same sprint speed and jump height, same carry limit, stamina, health, and mana. All the seeming choice in the game has no effect, and thus very little point, besides cosmetic ones.