That was a frustrating choice to make.
Terraria
Left 4 Dead 2
Pokemon (especially Ruby and Sapphire)
Final Fantasy Tactics Advance
Sonic Adventure 2: Battle
Sonic and Allstars Racing Transformed
Metroid II: The Return of Samus
Portal 2
Minecraft
And especially MapleStory. The main reason why it's addicting lies in one tiny little detail