Ooof, hard question. For a start, I'm not entirely positive what you mean by this gen as I'm a PC gamer - I assume you mean anything since the dawn of PS3/360, aye? I'll go with some general things as a game from a few 'categories', cause my actual top 5 would be pretty boring (Skyrim, Oblivion, both new Fallout games with preference on F3 would already make the 4)
Well, Morrowind falls out of this gen, so as a TES fan, I'll go with Skyrim as I prefer it to Oblivion (won't name it, but it's there as well). Predictable, I know, but hey, I just love me some TES games, giant worlds are really my thing. F3 actually comes pretty close here as well (yes, F:NV had better gameplay, but F3 was just awesome story/experience wise)
A shout out to Portal as well purely for the sake of originality. Loved the concept, excellent execution. P2 was great as well, but the first game was more groundbreaking (although I really liked the addition of more story in the second game, makes for a fantastic game to introduce people to gaming with)
Bastion wins as my indie title. Really blown away by it, I'm not an indie games person and Bastion really sucked me in to try a lot of indie titles. The game was simply beautiful though, every single piece of it, graphics, music and of course the narration. Even the gameplay was pretty fun.
I'll jump to Battlefield 3 as my multiplayer/FPS game. It's not perfect, but I'm a massive fan of the old Battlefield games and it was nice to see the series make a comeback. It's executed really well and I've gotten a lot of hours of gameplay out of it and plan to get more once Armored Kill and maybe some other DLC hits the shelves (CQ ain't really my thing, as I've mentioned before, I like big open maps and the reason I love the BF series so much is the dynamic gameplay that comes out of large open spaces and vehicle warfare).
Mass Effect 2 was my favourite in the series. It's the first one I played and I played it a while after it came out, I actually tried it while I was out of games and browsing through Yahtzee's old reviews looking for something interesting. Loved every bit of it, though I wish I played through the original game first, it took me several attempts to get over the lack of all the improvements the second game brought in.
Oh that makes 5. Damn. Well screw it, I wanna throw out one more title that I just have to put here. Consider it a buffer to choosing all the popular titles, I wanna throw out something I think people should try.
Alpha Protocol. Yes, the game had issues. The story was standard (fun, but not exactly a masterpiece), graphics and level design reeked of a game made some 10 years ago, combat and leveling, while fun, were really unbalanced and the minigames had an infuriatingly steep difficulty curve, starting from easy and ending up at "lol, hope you brought some EMP charges". However. As I've mentioned, the story and combat are fun. The stealth gameplay feel like old Splinter Cell games like Chaos Theory with some RPG elements throwing awesome powers in. And most importantly the dialogue system. It really needs to spread to more games, the dialogue in that game flowed so beautifully and made for such an amazing experience, I just couldn't be impressed with any other dialogue system in other games like DA, ME2, Skyrim etc. The dialogue options actually had an effect on both the current conversation and the future gameplay and it really felt like a part of the game and not something that was tacked on. Can't recommend it enough, it's a diamond in the rough.