I actually feel Link to the Past is a good start for Zelda since it's basically the bridge between the 3D Zelda's but still pertains elements of classic Zelda. I feel it's the most user friendly entry that still gives you a good idea concerning the whole franchise.

Mario I feel really should begin with Super Mario Bros. because it still incredibly fun despite it's simplicity. Definetly play this franchise in order.

Metroid has not fared as well as the other two. The first was incredibly novel and incredibly annoying if you didn't know what you were doing. Most people forget about Metroid II on account of it being for the GameBoy and Super Metroid is considered to be the best entry that follows the series formula. Metroid Fusion isn't a bad entry either if a bit more story heavy than fans are used to, not to mention it basically feels like a watered down version of Super Metroid. Metroid Prime Trilogy gets a lot of praise from fans but remains a bit divisive since it was a bit of a radical departure for the franchise but at the same time kind of revived franchise since it took a major hiatus in the last half of the 90s.