There are currently seven games in the main series: Metroid, Metroid 2: Return of Samus, Super Metroid, Metroid Fusion, and MetroidPrime, Metroid Prime: Echoes, and Metroid Prime: Hunters on the Nintendo, Gameboy, Super Nintendo, Gameboy Advance, Nintendo DS, and Gamecube. There is also a spin-off virtual pinball game. Of these, the most well-known is arguably Super Metroid, which greatly expanded on the nonlinearity of the previous games (Metroid 2 was disappointingly linear, though Metroid itself was not so bad), rewarding creative thinking and a heavy use of "secret" techniques. Super Metroid is considered to be one of the best games available on the SNES, in a class with such games as ChronoTrigger, FinalFantasySix, and LegendofZelda? 3.
Metroid the original was a very early game for the NES, coming out in 1986. The graphics are not exactly stunning, but they get the job done. Samus's range for upgrades is rather limited, consisting (IIRC, someone correct me if I'm wrong) solely of missile expansions, extra energy tanks, three different beam weapons, and the Varia suit, the screw attack, and of course the morphing ball and bombs. However, the exploration theme is already heavily present, with relatively huge world to explore. Unfortunately, because of space constraints, much of the huge world is just "Repeat this series of vertical platforms N times and put doors here, here, and here", which makes sightseeing less a possibility than it is in later games. Oh, well; it's still an excellent and rather difficult game.
The original Metroid was later reborn in 2004 as "Metroid: Zero Mission" for the GameBoyAdvance?. Basically, it had the same plot, map, and major bosses (sort of) as the original NES game. However, they spiffyfied the graphics and sound, added a map feature, reworked the terrain and puzzles to be more interesting, added some mini-bosses, gave Samus some abilities of the later games, and threw in a fairly unique surprise chapter just to top things off. All in all, an excellent remake, even if they did make the game easier. Also the only game in the series that didn't make me hate the wall jump.
Metroid 2 is, in my opinion, the low point of the series. Although it made a huge step forward in terms of graphics and Samus's abilities, it was limited by heavy linearity. The only way to increase the explorable area was to kill all of the Metroids in the currently explorable area. There is no backtracking involved unless you think you missed a missile pack or energy tank. Metroid 2 did, however, introduce the "Spider Ball" concept which allowed Samus to roll around on the ceiling, causing many fans to wail in disappointment when that item was not included in Super Metroid. Of course, the walljump is a better replacement. Right? AlexBobbs would like to note that he hates the walljump with the burning passion of a thousand suns
I don't know really anything about Metroid Fusion except that it looks spiffy. JeffBrenion, why haven't you gotten this game yet? And if you have, why haven't you shown me yet?
Metroid Fusion is probably best distinguished from the other Metroid games by its more extensive use of plot and dialogue. Instead of just getting dumped on a planet, Samus is investigating a space station and being guided by a computerized commander. In some ways this was interesting, and allowed for better sense of urgency and purpose. On the other hand, it seriously cut into the exploration theme of the series. Each time you enter an area, the computer guide draws out a map (granted, a very incomplete one), and tells you where you're trying to get to. There's still some exploration: you'll need to poke around a lot to get powerups, there are segments that don't require consulting the computer (although he yells at you about it), and the game seems to frequently block off the expected routes, which usually requires you to find some alternate secret path. In the end it's an excellent and slightly different entry into the side-scrolling Metroid canon, although fans of the old-school "explore everywhere" style will probably get a lot of ranting material out of this. But hey, you get hot Samus-on-Samus action to compensate.
MetroidPrime took the series from two to three dimensions, thus making it the one game in the series that the most people thought would be a flop. But it wasn't. An average first-person-shooter this wasn't. Definitely a superb addition to the series, featuring some of the best graphics seen on the Gamecube to date, more massive worlds, and enemies with actual AIs. My primary gripes with the game are twofold: Samus seems slow to react, lacking that instant-turnaround ability existing in the previous games, and there are few hidden areas in the game in comparison to Super Metroid. The former is a problem that can't really be solved in the game's form as an FPS; twitch turns would make Samus hard to control. The latter, I suspect, was avoided because there's just far too much surface area to look at minutely for it to be reasonable to require players to find that one hole that can be bombed open (I except here the scannable areas, since most gamers will be scanning any likely spot anyway). Barring these two, the game is excellent, ranked as Penny Arcade's #1 game of 2002.
I just finished Super Metroid with the following equipment list: both suits, high-jump boots, speed booster, space jump, morph ball and bomb, and freeze beam. You'll note some missing items. Had I been willing to do lots of assy bomb jumping, I could have skipped the ice beam and the speed booster as well. Notice that my only way of harming all bosses aside from the first Chozo statue was by use of missiles, super missiles, and power bombs. So I went kinda nuts on collecting items (81% by the end). Sadly, the bosses were all pushovers; I'll have to try it again with fewer replenishable items next time. - ChainMaille
I'm pretty sure that you need the speed booster to get into the Wrecked Ship (specifically, through the 2x2? room with the water and the missiles). Do you have information to the contrary?
You may well be right. I read an online guide to finishing the game with 15% item collection, and the items that he got were: 10 missiles, 15 supermissiles, 5 superbombs, the charge beam, both suits, the speed boost, the morph ball and bomb, and three energy tanks. Given how minimalist this list is, I suspect that the speed boost is in fact necessary. The freeze beam, of course, is not.