By gamers? Bioshock or Portal or Heavy Rain or w/e they consider art..

By the mass market/Mainstream? CoD(provided Activision doesn't kill it), Wii Sports, NSMBW, WoW(provided Activision doesn't kill it), and Pokémon (Any of the DS ones really)

METAL GEAR SOLID 4: GUNS OF THE PATRIOTS
GOD OF WAR 3
THE ELDER SCROLLS: SKYRIM
LITTLEBIGPLANET
UNCHARTED2
HEAVY RAIN
CALL OF DUTY 4: MODERN WARFARE
GEARS OF WAR 3
HALO 1
Maybe Killzone 2
LIMBO
And I suppose some nintendo games, even though I dont like any.

Wii
-Super Mario Galaxy 1 & 2: Focus was placed on gameplay and the way the game plays with gravity--there's hardly anything else like it.
-Mario Kart Wii: Pretty much every Mario Kart is timeless. I can still fire up 1992's original Mario Kart and have a blast. MK Wii is an instant classic.
-Punch Out: Another pick up and play game. It may not have been worth $50 to the average person but, years from now when it's a cheap thrill, people will still love it like I love it.
-Wii Sports/Wii Play: They weren't cutting edge and the were pretty ugly to look at but the games are so simple and it's easy to do good at them, even when you don't have any talent. Most of all, they're just fun.
-Skyward Sword: Of course, I haven't played this one yet but Zelda games aren't renowned for their technical prowess. It's the puzzles and the dungeons. As long as the puzzles are unique from other games and they challenge your mind, I can see it being a timeless classic that we'll come back to again and again.

Xbox 360
-Dance Central: If someone has a working Kinect, Dance Central provides a unique and fun diversion--provided it remains unique....
-A World of Keflings: This game is fucking ugly, but by not trying to be realistic, its big draw was its gameplay. All you do is command your subjects to build a city. That's it. And its addicting as hell.
-Splosion Man: Hopefully won't go the way of "Zero the Kamikaze Squirrel", "Aero the Acrobat", "Bubsy the Bobcat", and "Plok". Splosion Man is a platformer that I think is worth remembering. Like a ton of other games (Super Meat Boy, N+, Fancy Pants Adventures), Splosion Man is an old school platformer done perfectly.

Playstation 3
-Little Big Planet 1 & 2: To this day, people still create with the old SNES game, Mario Paint. Yeah, better tools for artistic expression exist but wherever people can find a way to create, they will. LBP is this gen's ultimate creation tool. PS1 era games like Fighter Maker and RPG Maker seem to be forgotten relics of a bygone era but I think LBP will succeed where those two failed.
-Valkyria Chronicles: With an art style that mimics a water colored painting, a simple yet epic story, and gameplay that centers on out-smarting the computer's AI, VC is as close to a sure thing as I can imagine. This may very well be the perfect video game. When the luster has worn off of your shiny new cutting edge game, VC will still be there challenging new generations with its indescribable charm.

Multi:
-Guitar Hero/Rock Band
-Puzzle Quest
-Braid
-Limbo
-Castle Crashers

This gen may be unique because, unlike previous generations, we're getting pretty realistic character models, environments, and lighting effects. Yeah, we can look at earlier games and scoff at their imperfections, but look at a game from last gen like Resident Evil 4. It looks dated but it still looks good enough to draw you into it. If future game consoles don't improve in power to an amazing degree, maybe current juggernauts like Gears, Uncharted, Heavy Rain, and God of War will be to us what games like Super Mario 3, Street Fighter 2, and Tetris are to us now--that is, if their gameplay holds up when their graphics falter.

ITT - people saying games that they currently like

Actual answer

Mario Kart and Brawl will still be played with some decent frequency.
WoW will still be going, although it will be down to 1-2 servers/region, it will be full of people who have been playing it for ages, and will be getting pretty much no new gamers.
A few of the single player games will still get fired up every now and again, probably sold for cheap like N64 games are on the virtual console. Examples that would be capable of selling noticeable numbers would be Zelda, Mario, Halo if the price is low enough to take into account how short the single player is.

LBP 1 and 2 will not still be relevant. The reason for this should be self evident by comparing Mario Part to LBP 1 and 2. LBP 1 was irrelevant when LBP 2 released. LBP 2 will be irrelevant when 3 releases. The community will move on - it is fine to play Zelda by yourself, but LBP needs a large community. LBP 6 will most likely do well in 10-15 years time.

Wii Sports and Wii Fit will still be relevant since they changed the industry, but I don't think people will still be playing them. Super Mario Bros is pretty much perfect as is, but I think the improvements in motion controllers will make them obsolete.

Games that will actually get played? Probably the Galaxy games, the NSMB games, Zelda, and Metroid Prime Trilogy for Nintendo.

Games that will age quickly? Most games dependent on an online community and games where realism is a big selling point. So any FPS with a weak single player or no local multiplayer will be forgotten quickly.

personally i think games that are very accessible and have a broad appeal will be more memorable in general than games that are perceived to focus on stuff like high production values and visuals...
so i guess my answer would be that games like uncharted and gears won't be as relevant when say in the next few years new consoles are out with higher standards are out... on the other hand i think games like wii sports, mario platformers and CoD will be more relevant
but funny enough for me personally the types of games i believe will be more relevant aren't really games i would consider to be better than games that will probably be less relevant for example i consider gears and uncharted to be better than wii sports and CoD

I don't think anything too dependent on motion controls will be anything relevant in 15-20 years. My guess is that they will feel too outdated given the superior motion tracking technology we will use in the future. Better kinect with better body motion capture will turn Dance central in a beta made for Dinossaurs. Same with the kind of Super Mario Galaxy. The lag in the controls will be way more noticeable.

Anyway. Whatever the developers feel they like to give us is my answer. This gen for example we had a crapload of Sega genesis games. And if games that always were crap, like Comix Zone, can be praised as good games, then anything from this gen can do the same in 15-20 years no matter what.

Also this one. +1

For me personally i don't really see games like wii sports, brawl or wii kart being very popular in years time. Are motion controls not a major part of these games, i would assume motion controls in 15-20years will make these games unpopular. Motion controls and graphics will continue to develop and games that release the same game each gen kinda kills one from the other gen.

Unless they change the way the game is played. FF13 plays nothing like FF6 for instance.