Hmm, good point about the playing styles, but I think that what matters the most is the weapon's potential, just because not many people use C.Shot it does not make it a worse weapon. Still, in the end all of this is a matter of opinion anyways.
And out of curiousity, how would the 1-3's weapons compare to 9's?
Potential is as potential does. Used effectively even a so-called lame weapon has the potential to be really useful. But that doesn't really mean they're all usefull, right? In a pro's hands Air Shooter is awsome (like I said) but I have never used it.
Anyhow, I'll take a crack at the weapon comparison. It'll be tough since I haven't played MM9; I can't say which weapon I would find most useful.
MM1
Cut - First weapon I get; most used by me in MM1. Many minor enemies are one-hit kills with the cutter and it has range.
Guts - It's forced to be useful in Wily's Castle, Elecman's level (to get the M. Beam), and against "Bubble Boy".
Bomb - Most useless weapon in MM1 that isn't forced to be useful. Good against Guts but not against much else.
Elec - Range + three directions at once + pause trick = broken weapon and best against the Yellow Devil. Great weapon in tight spots.
Fire - Another great weapon in tight spots thanks to the sheild. I use this one whem Cutter runs out.
Ice - Great against Big Eyes sicne it freezes them in midair. You can use this in Wily 1 to get past that "outside" part easily. Great against the fire towers; has the potential to make Fireman's stage easy.
win - 5 / 6 (with Guts' weapon having forced usefullness)
MM1 has some losing weapons (bomb and guts), but even guts is forced to be useful, so it isn't a total loss. However, the weapons we're left with are pretty powerful. They're very good when playing the game.
MM2
Metal - Best weapon ever. Used constantly.
Heat - Key to win against *some*bosses. But it's charging drains so much energy that I, personally, never use it.
Crash Bombs - forced usefullness against Piko Kun. Forced usefulness as a blockade-destroying device (especially in heat's level).
Flash - Useful in Quick's area. Useful in Heat's area (freezing the tellys). In fact: useful wherever tellys are.
Wood - I never use it, but it's useful in Crashman's area and in Wily's area (with the platforms). Useful against Airman. Useful against anywhere with Pipis. Generally usefull to people who aren't really me.
Bubble - Forced usefullness against Wily. Useful against most ground-trodden enemies. Not a very useful weapon tho.
Air Shooter - Surprisingly useful if used correctly against all enemies directly above you.
Quick - I never use this weapon except against the dragon and Guts Dozer. Not very useful. Too much like the buster, not enough range. Easily wastes out.
win - 6/8 (with Bubble and Crash having forced usefulness)
MM2 is designed so that it's weapons are useful in parts of the game. As such, each one has it's time to shine even if some aren't used as often as others (bubble). Even if Bubble loses out, it still stands out in our minds, so I can't say it's a total loser. The rest are also very solid. Rather balanced. They're all made to be used a certain way in a certain situation.
MM3
Shadow - I actually don't use the Shadow Blade very often. Forced usefullness against Wily 1 boss. Useful against Gamma (VERY usefull). Still, too similar to arm cannon and not enough range. However, it CAN hit those frogs in Shadow's level that was previously mentioned. Others do seem to find it useful, even if I don't usually.
Magnet - Most useful weapon in MM3. Good against the stationary subboses.
Top - Useful against Gamma, Shadowman, Mega Clone, and at least one Doc Robot. Hidden usefulness.
Hard - Good against any number of fortress bosses. Good against the subbosses. Good for removing blockers. Useful.
Gemini lazer - Brings Needle down to size. I found myself using this a lot in my childhood because of it's range and ability to hit things above me. But time has worn on and I don't use it too often.
Neelde - I always find myself button mashing more than using Needle, but it actually uses very little weapon energy. Has the potential to be a much better weapon than I usually use it as. Good against the Docs. Semi-useful.
Spark - Freezes the big "jumpers" in place, so it shares that use with Ice Slasher in MM1. Still, it's too much like Arm Cannon.
Snake - Forced usefulness against Gamma if you used up all of Top or just don't know about that weakness. Good against the top-spewers. Good against the turtle enemies (Gem's level) and most other ground-based foes. I'd say it has hidden usefullness, but isn't really used too often.
win - 5/8 Weakly tho, Magnet is so useful it overshadows other weapons when I play.
Each weapon in MM3 is pretty good all in all, with Spark and Neelde losing a bit since they;'re so like the regular weapon. Top was fun as a kid because it was so unique. People never think to use it, but it does have it's uses. Snake's weapon also has hidden potential. This game was designed to be beaten with the arm cannon if possible, however, so only a few weapons get a chance to truly shine. Still, each one has it's strong suit. With the exception of Spark and Needle, each one is memorable.
MM9 (and brear in mind I watched only vids)
Plug - Most useless weapon in MM9. Has some uses, but mostly useless.
Hornet - Nice for average players to get powerups. Wins for that.
Concrete - I don't see myself actually using this weapon much. However, speedruns show this weapon as having hidden win.
Splash - Powerful and broken, but shoots straight ahead. Good for some key opponents, but I dunno how many others. Still, pretty useful from what I've seen.
Galaxy - Key for those bombs in Splash's level and against the dragon and the Devils. However, I see this used in only those places and not many others. Still pretty useful.
Tornado - I don't see this as having any more or any less win than Gravity Hold. Forced usefullness in Wily 1. But not really useful.
Magma - Inafune says "finally, the charge is a gimmick again". Useful against Hornetman, but I haven't seen it do things that Splash's weapon can't.
Jewel - possibly the best weapon in the game.
win - 5/8 I've never seen Plug or Magma used too often. Tonado is useful, but not overly much it seems to me.
MM9 tries to mimick MM2 in that each weapon is useful in a specific situation. It also purposefully breaks certain weapons. Liek MM3, this leaves some weapons to be overshadowed. However, I don't see magma or Plug as being especially useful. Tornado seems good in a jam, but that's true of most special weapons in general. If we stack them up, I think MM9 is mostly impressive because of the neat visual effects they put in to make the weapons more memorable (sucking the flames off the dragon). Objectively speaking, if this came out right after MM8 I don't think people would be all that wowed by the weapons.