Mega Man 9

1. Galaxy Man
2. Jewel Man
3. Plug Man
4. Tornado Man
5. Magma Man
6. Hornet Man
7. Splash Woman
8. Concrete Man

Galaxy Man (Concrete Shot)
Jewel Man (Black Hole Bomb)
Plug Man (Jewel Satellite)
Tornado Man (Plug Ball)
Magma Man (Tornado Blow)
Hornet Man (Magma Bazooka)
Splash Woman (Hornet Chase)
Concrete Man (Laser Trident)

I found that it was much easier to defeat Concrete Man first, as his attack patter is pretty basic, so...

Concrete Man
Galaxy Man
Jewel Man
Plug Man
Tornado Man
Magma Man
Hornet Man
Splash Woman

The best order really comes from whom you find the easiest to buster right off the bat, so generally I would say your call. Now if you would like a personalized view: CthulhuKnight has the best choice set up

I don't know, but kill Galaxy Man first. He's a weakling (kinda).

Personally, any attack order where you have the Rush Jet before fighting Tornado Man.

Galaxy Man
Hornet Man
Splash Woman
Concrete Man
Jewel Man
Plug Man
Tornado Man
Magma Man

I recommend this order because:

- Gravity Man is fairly easy to beat with the Buster
- Use the Black Hole Bomb to suck up Hornet Man's bees and use Buster
- Use Buster or Hornet Chaser on Splash Woman, then follow the order for using each other bosses' individual weaknesses.

I like to take Hornet Man out sooner rather than later so you have access to the Hornet Chaser earlier on in the game to get power ups. I also recommend getting the Concrete Shot before tackling Magma Man's stage.

Every boss in the game has it weakness so i reccomend this order to make your battles easier:
Boss/Weakness
Splashwoman/ M. Buster, H. Tracker (tracker make easier to hit her while she is swimming up, both make a bit more damage than usual)
Concreteman/ L. Trident (destroys the concrete blocks)
Galaxyman/ C. Shot (freezes and stop the black hole)
Jewelman/ Blackhole B. ( absorbs the jewels)
Plugman/ J. satellites ( bounces off plug balls)
Tornadoman/ Plug ball (extra damage, and very helpful because tornadoman will most of the time be on the walls or the ceilling)
Lavaman/ Tornado blow (sends flying away fire shots)
Hornetman/ L. Bazooka ( extra damage and make easier to hit hornet trackers)

Also, every shot make more damage on the boss that is weak to it.

Sorry for spelling bad Magmaman name (lavaman, oops!). Also, is Magma Bazooka (M. Bazooka) not Lava Bazooka (L. Bazooka).