Firstly, I understand there's no such thing as the perfect game, at least, not for all. I'm not even sure that's possible even for me but I don't look at a 10 quite like that. For me, at that given time, I give a 10 if the game surprises me, keeps me enthralled (both story and missions/extra content) and I have a hard time stepping away or find myself going back for more. I've played a lot of games over the past 30-odd years and very few would garner a 10 from me. The first Super Mario and Contra for the original Nintendo (although they are prohibitively hard for this day and age) come to mind, as does Driver 2, GTA 3, GTA Vice City, (actually gave V a perfect score too) Fable, Elder Scrolls Oblivion AND Skyrim, the first Assassins Creed.
And Mass Effect 3. I made the mistake of playing this installment first as I'd only heard about it, knew nothing about it. Talk about BLOWN AWAY...the only thing missing was the ability to pilot the Normandy underhand- but it wasn't a huge deal for me as there was so much to do w/regard to exploration both on and off-board the ship. Beautifully nuanced, well written with a stellar cast there wasn't anything I didnt like, save perhaps that it needed to end at some time.
So, what'd I do? I did it backwards, and had a go at ME2, then went to the original to see how it all started. Then I went back and replayed 3 from the beginning, with a much greater understanding of the character development, the story, the reason d'etre if you will...it made for an even better experience the first time and in some ways, it was like I was playing for the very first time. It was like Star Trek and Star Wars, the best bits of both without the shortcomings. Oh, there were shortcomings, just none that come to mind which means they weren't a big deal for me.
Yes, the ending left somewhat to be desired as it seemed rather abrupt...I remember having the same feeling when I completed Metro 2033 for the first (and only) time; and again after Metro: Last Light. With so much OOMPH in the game, the ending was sort of a let down, yea? Still a fantastic game- which ending will be canon, I wonder? For mine, I speculated that Shepard survived- wouldn't mind stepping into his shoes again.