In what order should I play the Metroid games?

[PAOerfulone]

Start with the 2D games, specifically Super Metroid. Then from there you can go to either Zero Mission or Fusion, it's up to you.
Hold off on the Prime Trilogy until after the 2D ones.
The Prime games, while great, are an entirely different and gargantuan beast compared to the 2D games.
You will need to be in a certain mood and mentality to play those games. Especially Prime 2.