MM6 (which everyone agrees is the worst game in the NES series).
I don't.
Saying it has the "best selection" is something you'll have to prove game VS game, to get an agreement from me.
Well I did say IMO didn't I?
I can't compare the weapons in MM9 to the rest of the series because I'm the kind of player that prefers to go buster only for stages, so I'll just write why I think 9's weapons are good.
Laser Trident: Fast; you can rapid-fire it; pierces EVERYTHING; uses low amounts of WE and it's actually quite strong. If there's anything that's close to Metal Blade, it's Laser Trident.
Black Hole Bomb: One use of it kills almost all enemies; you can control it's direction and when it detonates; absorbs all projectiles; stays for a while absorbing anything that approaches it until it disappears.
Tornado Blow: Inmediately clears the screen of almost all enemies; makes you jump higher which can be useful for saving time in stages and also make some parts easier.
Concrete Shot: Quite strong; uses a decent amount of WE; creates platforms that unless you watch speedruns you'll have no idea of why they're so useful.
Jewel Shield: Unarguably the best shield ever in the series so far.
Plug Ball: Uses a small amount of WE; you can rapid fire it; goes along the ground and walls which makes destroying some enemies easier, faster and safer to destroy (for example those cannon things). It's pretty underused IMO.
Hornet Chaser: Chases the enemy, and if it is destroyed and does not leave an item it will go to attack the next enemy; gets you the dropped items without having to go back/out of your way to get them, and also grabs any items the stage has already (except the tanks I think); great for safely destroying enemies that are out of reach or in a tricky position.
Magma Bazooka: The 3-way shot makes it easier to hit multiple enemies; charged Mazooka is quite strong and does not use as much WE as Atomic Fire; if you position yourself correctly while shooting the enemy will get hit by the 3 shots and receive double or triple the damage,
this goes for bosses too.