Trine

This game has a lot going for it.  Right away, the game brings you into the story and explains the gameplay element of switching on the fly between a knight, wizard and thief.  Immediately upon starting the game, you'll notice that the visuals are very appealing, particularly for a downloaded game.  While the game is played as a 2D platformer, the backgrounds are displayed in gorgeous 3D.  The environments are colorful and show a variety of fantasy elements, including forests, castles and dungeons.  
  
What starts out as a fairly simple platformer becomes more and more complex the further you delve into the game.  Simple jumping puzzles and skeleton battles become much more intricate by the end, and the in-game physics engine (while not perfect) provides for lots of entertaining areas. 
 
The different abilities of each of the characters provide for multiple ways to solve many of the puzzles.  The knight is generally a combat character, but can be slow and unwieldy to use.  The thief has a grappling hook and moves and jumps well, but is a weaker combat character (but still probably the most useful character for the majority of the game).  The mage starts out quite weak, with the ability to summon a single object, but by the end of the game becomes very useful. 

The upgrading system deserves particular praise.  While the new abilities of the knight (an additional weapon, more damage) and the thief (fire arrows, additional arrows fired) are solid, the wizard's upgrades are probably the biggest improvements.  In the beginning he is able to summon only a single crate, but by the end he can summon a variety of crates, planks and levitating platforms that allow for great opportunities for the player to be creative in solving puzzles later on in the game.
  
The audio is also generally very solid and frequently entertaining.  While not spectacular, the voice work (particularly the narrator) is well done and rarely over the top.  Music is consistently solid and ties into each level well. 

Despite the praise I heap on it above, Trine is not a perfect game.  Certain aspects of the game are not visually stunning.  For instance, pull up the upgrade menu, and it's almost like you're looking at something from a different era.  Suddenly the colors aren't as crisp and lines become fuzzy.  In addition, the gameplay isn't particularly novel or creative.  The story is good, but not groundbreaking, and the 2D platformer genre has certainly seen many previous entrants.  But what makes Trine stand out is that few have done it so consistently well.