Can't defeat Airman either? Well, not untouched anyway... stupid tornadoes!

I haven't come across a totally useless weapon in MM2...
Bubble Lead kills the spring heads as well as the rabbits (2-3 hits) on the "stairs" in Wood Man's stage.
A single Crash Bomb kills Blocky in 1 hit on difficult mode (not to mention hurting no less than three bosses with the explosion - Quick Man, Flash Man and Wood Man; the latter only if you fire into his Leaf Shield, which is entirely counterproductive with any other weapon). The only annoying thing about the Crash Bomber is the fact you only get 7 of them, unlike Bomb Man's bombs in the previous game. On a side note, the explosion is slightly larger in the Wily Wars remake.
Time Stopper skips 2 out of the 3 "Hotdog" mini-bosses on Wood Man's stage as well as the Quick Man lasers (if not used against Quick Man himself), Metal Man's spikes and moles, the falling crabs at the end of Bubble Man's stage...
Air Shooter takes out both the Sniper Joe mechs and the Joes themselves with ease, not to mention Tellies.
Quick Boomerang can be used on almost every single thing, even the above Hotdogs and mechs, and uses practically no ammo. Somehow though, it can't break a Pipi egg... If only the auto-fire rate was about twice the speed, it's way to slow to be useful (same with its "successor", the Needle Cannon).
Leaf Shield can take out moles, Pipis, Scworms and somehow works just as well on things underwater in Bubble Man's stage (just don't hit the Anko with it). It is pretty useless against bosses though, Air Man aside.
Metal Blades are way overrated. They can't touch the Sniper Joe mechs, Quick Man or Crash Man and barely damage any of the Wily bosses (if they don't bounce off to begin with). About the only thing they are useful on is the entirety of Bubble Man's stage, which is an easy level anyway (the only dangers are the bottomless pits, the falling platforms which drop really fast, and the underwater spikes).
Yes, I left one weapon out, take a guess. Maybe I did find a weapon I don't use beyond destroying Wood Man quickly and the front of Dr. Wily's machine.

Also one interesting thing with the boss rush:
If you pause the game, it shows the weapons in the order of P, H, A, W, B, Q, (next page), F, M, C, 1, 2, 3
From top to bottom, the boss order is exactly the same - Heat Man, Air Man, Wood Man, then Bubble Man and Quick Man on the bottom, and then Flash Man, Metal Man and Crash Man on the right hand side.
H F
A M
W B Q CBack to annoying things:
MM1: The fast-moving flames in Fire Man's stage, which require perfect timing to get through unhurt (not counting the Magnet Beam). I am still yet to dash through them without getting hit by at least one of the two.
Wily Wars: Elec Man apparently ate a Crash Bomb before you entered the boss room and he now runs and jumps all around the room at ludicrous speed. You thought giving one to Heat Man was bad...
MM3: Junk Golems, they are still a pain to avoid without losing a chunk of health before the boss rush.
MM4: I still don't like Metall Daddy's oversized, square hitbox, and no weakness (Rings or Dust, your choice, they both deal 2 damage). Why is it that the Wily Machine from MM6 is easier to get underneath and defeat with less damage done compared to this Stage 1 boss?
MM7: The fact that you
must "use the flame weapon in the woods" to find Beat; the ladder itself physically doesn't appear until you do so (it's not just hiding behind the tree, unlike the Rush plate in Cloud Man's stage, which is still accessible without changing the weather). Not only that, but it isn't even the Beat you were hoping for! Now he's only there to save Megaman from his own poor jumping skills. Also, I don't like the fact that you have to find three instances of Proto Man in one particular order, so you can defeat him to obtain his shield. Not that it's required though (it's just one of those X series "pixel hunt" elements which made its way into the classic series, although technically it really started with the hidden Balloon and Wire items in MM4 anyway).
I have no comment on the later series, I pretty much dismissed them after the Super Nintendo. Once you bring in "cyber elves" (Zero series) and Pokémon style gameplay (BN), I'm out of there. Also, the fact that Capcom moved towards handhelds (GBA/DS/PSP) also turned me off the series as a whole.