The Witcher 3 and it's not even close.
MGS is favourite series ever, I've been playing them since the beginning, but I had nothing to complain about when it came to The Witcher 3. It has a better open world, with much more life and a lot of variety in the environments. MGSV feels empty in comparison and there's not a lot of variety. The Witcher 3 has a satisfying story with unique handcrafted side quests that all have their own stories. MGSV has repetitive side quests which mostly feel like padding and unfortunately it suffers from cut content which leads to a very unsatisfying conclusion. MGSV also has the worst story of the all the MGS games imo, maybe Peace Walker is slightly worse, I don't know.
With that all said the actual gameplay of MGSV is really good and the game runs really well on the PS4.
ed: I think The Witcher 3 has ruined open world games for me, it's almost too good.
Sorry Kojima.