I think I've yet to complete a game...the only ones I've finished that I can see at the moment are Portal 1 & 2, Mirrors Edge, Fable 2 (would like 10 hours of my life back), Halo 3-Reach and the original monkey island GotY edition...I have completed more, I don't think there's a main pokemon generation I haven't completed...
See when it comes to games I tend to play half way, then get something else and if the story was REALLY engaging then I just youtube it on a Let's Play. I get bored easily and once you've played a game half way through, it doesn't really change in mechanics except for plot or a possible boss fight. The only exceptions will be when something genuinely keeps my attention for more than 2 hours (ahem Portal), or if the games so short that I can complete it first day (Halo, Mirrors Edge, Monkey Island etc.)