This is an easy one: Luigi's Mansion 2. I remember wasting my entire weekends replaying the first one again and again and again. Although the story stays the same the sixth or seventh time I played it, the gameplay was so amazing I hardly focused on the story in my later playthroughs. If I had to guess, I would say I have beaten Luigi's Mansion at least 20 times. It is one of my favourite games of all time, and thus I am very excited for the sequel.