My introduction to Metroid was....Metroid. Metroid 2 was a good follow up and Super Metroid perfected it.

Zero Mission is a pretty good game and Fusion is a total disaster from top to bottom. A complete train wreck!

Metroid Prime 1 and 2 are great (as long as you turn off hints/help).

I wish Nintendo would make a gorgeous 2D Side Scrolling adventure as a true follow up to Super Metroid but after Fusion and Zero Mission, I doubt they know how to anymore.