In a sense, Metroid is one of the greatest games of all time. Just not in the sense that you'd still want to play it.
Metroid is a side-scrolling action-adventure game in which you play as Samus Aran, a sleek, agile bounty hunter who has been dispatched to the planet Zebes to defeat the metroid menace. You start off with nothing but a weak little blaster, but as you explore Metroid's environments, you'll find numerous power-ups that add to your arsenal and your repertoire of moves, as well as increasing your health. This sort of exploration and character building was largely unheard of at the time, and it made for a very satisfying experience. You could argue that in this regard the influence of Metroid is still being felt today.
But satisfying though it was back in the day, so many advancements to games of this type have come along since that it's hard not to find Metroid somewhat frustrating today. For instance, there's no automap feature. At the time, many players would bust out their well-worn Official Nintendo Player's Guides to consult its maps of Metroid's environments, but I don't suppose you have one of those lying around. Well, there are plenty of FAQs on the internet. Or you could just play through the game on your own, but be prepared to spend lots of time putting bombs all over the place just to see if blocks happen to break, allowing you entrance to a new area.
There's no doubt that Metroid does a lot of things very well. Samus' unique abilities, such as the morph ball and the screw attack, are still tremendous fun. Samus' animation is still somewhat impressive, although the way she runs looks a little goofy. The battle with the game's final boss, the Mother Brain, is an iconic video game face-off. And the game creates a sense of atmosphere and isolation that at the time was unparallelled. The music plays a large part in creating this ambiance, and the tunes are still very effective and memorable. But Super Metroid, the game's SNES sequel, improved on Metroid in pretty much every way, so you may want to save your Wii points for that game's forthcoming Virtual Console release rather than troubling with the original. And if you really want to experience the story of Samus' initial adventure, you'd be better served by tracking down a copy of the terrific GBA remake, Metroid: Zero Mission, which applies all the advances the series introduced in later games to Samus' first contact with the metroids.
If you played Metroid back in the day and want a trip down Nostalgia Lane, you may find that it was more impressive in your memories than it appears to be in the cold hard light of today. If you've never played Metroid before, the remake is a much better option. Metroid is an important part of video game history, an extraordinarily influential game, and in that regard it will always be great. Indeed, Metroid is, in a sense, one of the greatest games of all time. Just not in the sense that you'd still want to play it.