Hard choice Minecraft was a phenomenon on YouTube bringing creative mechanics to content creators, however I don't consider it a notable game of 2011 as it was already played a lot before in beta.
Portal 2 is perhaps the best all-time puzzle game, everything is perfect, level design, gameplay, voice acting, but I think it was less impactful than the other 2 that remain to be mentioned.
Skyrim improved everything in relation to Oblivion, raising the bar for Western RPGs and OW games
but for me the winner should be dark souls, it revived the sense of difficulty and reward of games from the 90s, exceptional level design of connected areas, memorable battles, brilliant use of online, as well as things that transcend the game itself, it created a subgenre, since then its formula has been widely copied, in addition to having influenced dozens of games, cultural impact on the industry, it has become synonymous with difficult games , at some point everyone must have seen something like mario something is the dark souls of the mario series. After thinking about it, it's actually an easy choice.