I regret making this post
Seeing as Pokemon is a rather obvious choice, I'd go for Animal Crossing here.
The concept of the game sounds rather boring, but the interactions with the villagers is quite enjoyable and sometimes even funny. Being able to create a house with lots of furniture is rather addictive too, especially if you aim for the best scores from the house rating system.
Resetii is also one of the funniest characters in gaming history, so sometimes I turn the game of without saving just to see him!
Overall, it's a great series of games and I can't wait for the heavily delayed New Leaf to be released.