Portal 2 is a perfect game. There's a perfectly reasonable statement. And it's my aim by the end of this rambling collection of hyperbole to convince of it's validity. 
 
Portal 2  is perfect largely down to the excellent gameplay and art design and dialogue etc but personally I class this game as perfect because I feel Valve achieved exactly what they set out to achieve. Portal 2 has no tangible weaknesses. It's gorgeous to look at, smooth as melted butter to play, fiendishly clever, exhilarating and rewarding, written with a razor sharp wit and satire so dense you could bite it off in chunks. The voice acting is unparalleled with J.K. Simmons and Stephen Merchant supplying some of the most hilarious dialogue this side of Monty Python. It's just about the perfect length going on long enough to surprise you with how much fucking "stuff" there is but ending before it begins to feel drawn out. Also the ending is brilliant. 
 
Then there is the co-op. One of the best implementations of  duo co-op (du-op?) I've ever seen and easily the best since MW2's spec ops. I'm yet to finish the co-op section but it has all the charm, wit, intelligence and (so I hear) longitude of the solo game. The Art direction really shines in co-op with Atlas and Peabody being some the most expressive and beautifully grafted characters I've seen this generation. Coming from a source engine game that is significant. 
 
These are all perfectly good reasons to call Portal 2 and excellent game. But I claim it is perfect. My reasoning behind this is thus: 

With Portal 2 I feel Valve set themselves a task which they met or exceeded in every facet of Portal 2. Everything came together in the pacing with the puzzles and the art design as mentioned above but here's the nub of it for me: nothing, not one solitary element of the Portal 2 experience feels as though it was not lovingly crafted. Nothing feels rushed or crammed in to save time. Portal 2 feels like a labour of love in every way. 
 
Let me give you a comparison. Lets compare Portal 2 to Mirror's Edge. Now I loved Mirror's Edge but if we look at every facet of that game something becomes apparent. The firearms combat lacks satisfaction and feels, here's that word: rushed. It's as if the developer at some point felt "that'll do" with the combat be it through laziness or time constraints or whatever. Nothing in Portal 2 feels rushed. 
 
Another comparison: Dead Space 2. More specifically the Dead Space 2 multi-player which was clearly shoehorned in to cater to EA's project ten dollar and was, objectively speaking, utter shit. Nothing in Portal 2 feels shoehorned in. Everything is there for a reason. Every virtual inch of that game was crafted with care and consideration. Everything included is to the overall strength of the product. 
 
Of scale of 1 -100 in terms of what Valve wanted Portal 2 to become I have to believe they score 100/100. What I mean by that is I feel that Portal 2 The Game is the perfect reflection of valves vision of Portal 2 The Concept. Nothing is lacking in Portal 2 from my perspective. I honestly cannot find a fault in this game. Obviously that's subjective but I also get the feeling (as I have read nothing to contrary) that this game is precisely what Valve wanted to release with no concessions or compromises.  This is why it is Perfect.
 
The only other game in the last ten years I can think of that also fits this criteria is Half Life 2.