For consoles, I'd have to recommend the Mass Effect trilogy on the 360. Say what you will about the third game's ending, the overall package still involves some quality world-building and some characters you grow to empathize with - that are a whole lot more than disposable Redshirts or handy statistics on legs or pack mules to keep along.
For the PS3, my money's on Uncharted 2. The pace is perfect, the humour is quite good if you've got a tolerance for quirkiness, and the whole feel of the game is one of self-indulgent Pulp goodness. It's like watching an Indiana Jones movie sequel that never was. Overall, it makes for awesome summertime entertainment.
For the feel-good value of these titles, however, I'd recommend anything and everything by ThatGameCompany. Jenova Chen's guys work together to create experiences that are approachable for non-gamers, intriguing for back-seat observers, and incredibly powerful for whoever's lucky enough to have a controller in their hands. Journey is something that needs to be seen to be believed, even if it's almost painfully short. Consider flOw as a great thesis of the studio's desired output and Flower as a slightly heavy-handed take on environmental responsibility that's still worth a gander for the beauty of what it presents.
For the Wii, nothing beats Super Mario Galaxy. The first one, not the second. This is Nintendo at its best and boldest, like it hasn't been in almost ten years. The game almost mercilessly whips you back and forth between a rising difficulty level and moments where you're allowed to just sit there and gawp at what they're able to pull out of the Wii's comparatively imferior hardware.
And the score, man. The score. I still get chills from the drum punches and the flute lines in Good Egg Galaxy's soundtrack. It basically screams for you to leave your cynical adult self at the door and rediscover the feeling of being glued to your controller for reasons beside scores or basic completion rates. Whenever I want to feel like I'm ten years old again, Galaxy is my choice game.
Seriously, Good Egg Galaxy is one of those songs I keep on an iTunes playlist called "Life is Awesome." I dare you to stay depressed after giving it a listen.
I know you mentioned the PC as being your weak link, but I absolutely have to insert Myst and its first sequel, Riven, as contenders to that role for their platform of choice. The Myst series triggers the same sense of wonder that Galaxy does - but in a wholly more mature and cerebral way. Instead of just thinking that what you're looking at is visually arresting, you'll find that it's all gorgeously put together - and that there's a purpose waiting to be found for nearly everything.
For the 3DS, the only thing I can really think of is Animal Crossing: New Leaf. It needs to be enjoyed in moderation, however, and with complete awareness that you're essentially playing a glorified Facebook game. What did me in was the art style and general presentation, seeing as I'm not what you'd call a Crossing veteran. Play in little chunks or in big ones, screw most long-term objectives and just have a blast.
