Weapon Swap Order

The quick weapon switch order is Mega Ball -> Flash Bomb -> Thunder Claw -> Tornado Hold -> Water Balloon -> Flame Sword -> Homing Sniper -> Astro Crush.  

Remember you can always press L+R to return to Mega Buster.

Mega Man 8 is unique in that quick weapon switching while a weapon is on screen does not cancel the existing weapon.  You can take advantage of this to combine several weapons together and ease platforming.