Top 5 games list is impossible, but I'll try. These are in no particular order.
Zelda Series (must I pick just one, each one is special and marvelous)
Tetris (I just can't leave out this game, it has shaped me!)
Super Smash Bros Series/Mario Kart Series (Included together because of equal amounts of fun. Similar to a certain extent, but hard to put to words)
Custom Robo/Kirby's Air Ride (Included together because of one aspect; multiplayer! Most fun I've ever had with friends via video games.)
Kid Icarus: Uprising (I genuinely have not been this impressed with and entertained by a game in a while. The polish, the details, the content, the FUN!)
Ehhem...Thank you.