This is yet another category that I'm going to answer with a favorite example rather than trying to pick an absolute champion. Each genre has its own standards for great gameplay, and I can't definitively value one particular form of awesome over all others. I'll just go with my most recent BEST EVAR series for knocking it out of the park.
When I first I heard of this series, I so hoped I would like it. Its post-apocalyptic role playing nature seemed promising, as did its snarky sense of '50s throwback humor. It also has quite possibly the best mascot ever. I picked up New Vegas on sale with my fingers crossed, and damn was I hooked. It hits so many Do Want buttons I never knew I had.
Gameplay is all about methodical exploration in a Wild West setting. It's never-ending in the good sense, with the joy of discovery apparent throughout the game. Missions lead to more missions. The world map has boatloads of optional content, from simple caves to underground vaults and buildings with their own character, content, and obstacles. The game tracks your goals and marks targets on the map, cutting down on tedium without hand-holding.
And you can't experience it all on one playthrough, partially due to the requirement of choosing to ally with a faction. In addition to your major allegiance, you have options for dealing with other towns, settlements, and smaller groups throughout New Vegas. This gives the feeling of making waves within the world, affecting it in your own way.
Combat can be played like a FPS, but it doesn't have to be. The assisted aiming system brings a focus on strategic use of weaponry - going for that headshot, or crippling limbs so the big bad Deathclaw must resort to limping after you. I'm bad with fast twitch precision aiming, so this system is ideal for me. The RPG element is also more about planning than reacting, giving me a chance to play the character I wanted by making informed decisions on how to finish different scenarios.
And if I'm allowed to go off-topic for a moment - HELL YES DAT SOUNDTRACK.
Day 2 - Your favorite character.
Day 3 - A game that is underrated.
Day 4 - Your guilty pleasure game.
Day 5 - Game character you feel you are most like (or wish you were).
Day 6 - Most annoying character.
Day 7 - Favorite game couple.
Day 8 - Best soundtrack.
Day 9 - Saddest game scene.
Day 10 - Best gameplay.
Day 11 - Gaming system of choice.
Day 12 - A game everyone should play.
Day 13 - A game you’ve played more than five times.
Day 14 - Current (or most recent) gaming wallpaper.
Day 15 - Post a screenshot from the game you’re playing right now.
Day 16 - Game with the best cut scenes.
Day 17 - Favorite antagonist.
Day 18 - Favorite protagonist.
Day 19 - Picture of a game setting you wish you lived in.
Day 20 - Favorite genre.
Day 21 - Game with the best story.
Day 22 - A game sequel which disappointed you.
Day 23 - Game you think had the best graphics or art style.
Day 24 - Favorite classic game.
Day 25 - A game you plan on playing.
Day 26 - Best voice acting.
Day 27 - Most epic scene ever.
Day 28 - Favorite game developer.
Day 29 - A game you thought you wouldn’t like, but ended up loving.
Day 30 - Your favorite game of all time.