Having played the last three games, I would say Skyrim.
While I do miss some of the old skills, the new leveling system is better. Its not perfect, but the statpoint allocation from the previous game after a lvlup was terrible.
I find the new crafting skills have somewhat odd balancing. After powerleveling blacksmithing and breaking the game with it I restarted my character and took a more normal approach, which worked out fine.
But this is all without any patches. And once the constructionkit is released things will become endlessly more interesting + custom balancing. (my immortal horse annoys the hell out of me)
For example: I want to be able to craft arrows. I want a non-enchanted Nightingale Armorset, because the default ones are terribad. Etc etc...
Nevertheless, my main problem with Skyrim is replayability... Its way too big!
This may differ from player to player. I personally like to complete a game as much as possible with a single character. Right now im level 35, 55h playtime, having completed only 2 of 5 faction questlines (Thieves and DarkBrotherhood).
These were awesome, but too short. I wish they had made the faction\guild questchains longer and less random quests all over the world. Not really wanting to do ALL the random sidequests, and not knowing which ones are the more interesting ones, I prefer doing multiple faction questlines. But they dont last too long, and they also dont go together too well lore-wise. Thief + DB makes no sense from an RPG point of view.
Completing the game a satisfactory amount would take like 200 hours playtime. Thats longer than my 3 playthrougs of Mass Effect 2. How the hell am I supposed to want to replay that. Maybe 2 years later with a few dozen mods
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Ok enough text.