As the world's biggest Valve fan, I can state that my biggest gaming sin by far must be that I still haven't played Portal 1 or 2...

It's okay. I didn't play Half-Life 2 until earlier this year. I think that counts as being far worse.

MGS peace walker. I own it but havent found the right moment to play it. Now I feel the urge to play it before MGS 5.

Yeah, that's just unforgivable!

Never played any Zelda, Shenmue, God of War, Ratchet & Clank, Sly Cooper, Metal Gear Solid, Mass Effect, Dragon Quest or Final Fantasy game. Half Life 2, Shadow of the Colossus, Uncharted 2 or 3, Super Mario Bros 3, Super Mario Galaxy 2, Banjo Kazooie.... and I'm sure theres many more >.

Metal Gear Solid, Souls series (but played BB recently and liked), Final Fantasy, Batman Arkham series, almost all Zelda games (especially 3D), Deus Ex, NBA 2k (since I'm a big NBA fan more than 25 years and a huge fan of NBA Live games on Mega Drive 2).
Soon I'm planning to play Dark Souls, FFX and BA Asylum, and Deus Ex HR. Don't know about whole MGS series, but sure I'll play the fifth soon. And I hope NBA 2k16 also.

Lol, where to start...

Of the big ones 'everybody praised', I've never played;
Grand Theft Auto V, IV put me off so hard I'm not going to give V a chance
Any Final Fantasy, unless you count Mystic Quest on SNES
Batman Arkham, except the one that's considered a bit mediocre but I actually like (Origins)
Any Metal Gear, including Solid
Shadow of the Collossus, the Dreamcast made me not get a PS2, contradicting the world in the process
Half-Life, I guess I didn't dare to because my mom erased it from her hard drive when it got scary way back when
Chrono Trigger, I live in Europe

And many others I suppose.

MP2 Echoes

I never played any Nintendo games.
But I don't consider it a "sin", but a wise choice, because all those games are very unappealing to me, I think that they are just a collection of cartoony mascot games aimed at the preteen crowd, and wouldn't play them for free, let alone buy an entirely different console to play them on.

I'm a fan of stealth games yet I absolutely loathe MGS games ...