#46 - Portal 2 (Kudos to Smeags for guessing this)
Portal 2 & I have had a tricky relationship. On the one hand, when I first played it, I felt totally ripped off having paid £35 to get a game that was only around double the length of the already-ridiculously-short Portal; with a co-op campaign that was fleetingly over after about an extra 2 and a half hours.
However, trying to quantify Portal 2 would do it a great dis-service. Portal 2 took everything which made the original great and just absolutely ran with it. The portal mechanic was greatly expanded and the new gels added a whole new level of interest to the puzzles. Casting Stephen Merchant to portray Wheatley was an absolute stroke of genius; never managing to upstage GLADOS' brilliance but bringing a whole world of awesome to the character himself. The world you explored was a fantastic take on the clinical simplicity of the first game mixed in with nature finally reclaiming the land as its own. Atlas and P-body's co-op adventure; although brief; may be one of the most satisfying things I ever had the pleasure of playing.
Pure brilliance from start to finish. In fact, I kinda wish I'd put it higher in my list now. Maybe next year!
Smeags correctly identified my clue-quote which actually summarizes the game pretty well I think. GLADOS; humour and thinking. A great combination.