Four years ago I would have said Skyrim, but since late 2015 the top spot has been held by Witcher 3. I have over 2600 hours clocked (I own the game on both Steam and GOG), and I've no intention of stopping playing it.
As for why... well, I'll just copy/paste my GOG review of the GOTY version.
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Since I am reviewing the GOTY version, I will leave out some details.
As I write this review, Galaxy shows 1128h 39m clocked. More importantly, I do not regret a single second I've spent playing the game. That right there is the strongest recommendation I could possibly give.
Pros:
+ writing
+ music
+ audio and voice acting
+ combat (including skills & abilities)
+ story with its multiple paths & end states
+ meaningful choices with consequences (familiar feature from the previous Witcher games)
+ general gameplay
+ humour, though often dark
+ graphics
+ top-class performance optimization
+ characters
+ Geralt is awesome and deserves his own bullet point
+ the game world
+++ the expansions
Cons:
- ruins any social life you may have, for at least a couple hundred hours
- can make other games feel "meh"
(The quote function works differently than what I'm used to. Interesting, but also inconvenient as I had to remove it.)