I played quite a lot of good games in '07 but if I had to pick one to win some kind of all-encompassing award for my most satisfying, rewarding and memorable experience of the year I'd probably have to go with Bioware's Mass Effect. (Disclaimer: have not played Super Mario Galaxy). Right off the bat I'll admit this game has some problems. Technical issues do crop up, there's some strange design choices afoot and you can nitpick the flaws to death if you choose to - I know I did during my first few hours playing through. But once I accepted Mass Effect for what it is, warts and all, I was sucked in completely and haven't looked back.
First off the universe created by Bioware is the deepest and, frankly, the best ever seen in video games as far as I'm concerned. Even minor species like the gelatinous Hanar and the giant, monotone Elcor are utterly fascinating with a rich galactic history and culture. There are many plots and themes at play here such as the relationship between organic and inorganic life, choosing between slavery and genocide, the different faces of humanity and questioning religion and our notion of God. Fortunately the game is not nearly as heavy handed as all of that, and you can just as easily describe the title as being Jack Bauer in space with plenty of action and some pretty high stakes. It's a space opera of the highest caliber with humor, drama, interesting characters, and a whopper of an endgame that kept me on the edge of my seat and left me feeling spent and satisfied when I was through. And even after that I had to make some pretty major decisions that stand to affect my character's story in the sequel in significant ways. There is plenty going on here that sets the stage for a second and third game but not in the Halo 2 sense, more like "I can't wait to see what X's homeworld is like" or an important antagonistic species that's yet to be seen up close.
Another short disclaimer, I love a good dialogue tree (big fan of PC adventures and all that) and if you aren't so keen on listening, watching and reading for extended spells maybe ME isn't for you. Sure the game is a highly capable third person squad shooter with fun powers and customization, but at it's heart is KOTOR-inspired RPG goodness. The outstanding dialogue and voice acting, beautiful character models that convey actual emotion,the choices you make, side quests, environments, everything a great open ended WRPG needs is here in spades but in a much more focused package than, say, Oblivion.
Best of all is the overall feel and look of the game. The whole late 70's, early 80's Sci-Fi thing going on here is just brilliant - think 2001 or Blade Runner. The music is perfect, Brian Eno-esqe synths and orchestral flourishes never get in the way but always set the mood just right. And the film grain effect (which can be turned off) really works, it's a nice little touch that may get overused by developers in the future but for this game I loved it.
So there it is. Game is buggy, autosave is broken, camera is lazy, planet exploration gets repetitive, levels are broken up by elevators everywhere, it takes a few hours and levels to get the hang of combat, there's only one real city in the game, menus are poorly designed. Plenty of room for improvement. But I got so swept up in the world and had so much fun on the journey that after a point none of that mattered. I'm playing through a second time as a different build and it's just as enjoyable as the first. It's a really wonderful game and the one that stuck with the most from a truly memorable year for gaming.