MGSV probably had the most polished gameplay I've ever seen in any game.
Even if the story fizzled out due to not having a real Part 3, the gameplay was so good it could be overlooked imo. It really set the standard in terms of how open world should actually be (freedom to perform missions how you want).
I agree completely with this post. MGS V gameplay is perfection, everything works exactly how a game should work down to the online components. Even the unfinished story can't tear down something that excellent.
My personal game of the decade though is Kid Icarus Uprising. Probably the most underrated game of the decade too. This is as perfect a Nintendo game experience as they come from the genuinely fun and well-written story and characters to the (again) pitch-perfect gameplay mechanics. For its style it is the perfect 3DS game but its on-rail shooter and third person action mechanics could work in any form. It is the game where Sakurai introduced the 'difficulty scale' mechanic (Fiend's Cauldron) later used in Smash Bros where you spend points to decrease or increase the difficulty and get extra awards accordingly, a natural and intuitive way of scaling the difficulty to your liking with a continuous push to challenge higher difficulties.
The game also features weapon fusing as a means of leveling up naturally by fusing earned weapons and combining strengths/weaknesses and special attributes to create the perfect weapon for your play style. The system is insanely deep with hundreds of possibilities, all starting with deciding which of the different weapon classes you want to use and what offensive or defensive powers compliment them. And the better your weapon, the higher the difficulty you can challenge to gain more rewards and power up your weapon further.
The result is a game with a story you could finish in probably a dozen hours or so, but I ended up spending over a hundred hours in because the game rewards you so much for challenging yourself.