I'm not a diehard fan of the Metroid series, and in fact I let it pass me by almost completely until I played Zero Mission last year. I had dabbled in Super Metroid a few times but it never grabbed me for one reason or other. Zero Mission grabbed me with both hands, and didn't let go until I'd finished it. Hell - coming remotely close to finishing any game these days is something for me, so this one did something very right.
I can only trot out the usual praises for the Metroid style: A player character whose abilities grow as she explores, thereby opening up access to previously inaccessible areas, a tremendously atmospheric and varied set of environments complemented by an evocative soundtrack, and finally a puzzle-solving aspect that covers the entire gameworld. The movement of Samus is so solid and satisfying that simply running around the levels is a pleasure - and actually alleviates what might otherwise be tiresome backtracking (though in the case of Zero Mission the back and forth is punctuated by plenty of new events and secrets).
This is my favourite Metroid game, which obviously discounts Super Metroid from appearing higher in this list. I prefer the controls and feel of Zero Mission, and that's enough to stand it apart because to be honest the Metroid template didn't change much between the two in terms of structure.