Air Man
(Mega Man 2)

They say that you never forget your first time, and I suppose they're correct.  Air Man was the very first Robot Master that I defeated all those years ago.  Why start with him?  The Mega Man 2 instruction manual told me to take him on first.  His non-humanoid design sets him apart from the other seven Robot Masters in the game, what with that big fan in his chest/mouth.  His weapon isn't entirely useful at first, but becomes invaluable in later levels of the game.  I always felt there was an untapped potential with Air Man, although I cannot put my finger on just why I believe that.  He just seems destined for something greater.

Cut Man
(Mega Man)

Cut Man shares the honor of appearing in more Mega Man games than any other Robot Master with Guts Man thanks to brief cameos in games such as Mega Man 7, Mega Man 8, and even Mega Man X8.  He's not all that memorable in his early appearances, and it's his Powered Up makeover in recent years that makes him stand out from his robot brothers.  His Rolling Cutter yields mixed results, but his wall-jump skill can be extremely helpful.  He's a rather trusting and polite robot, as it turns out.  He even addresses the blue bomber as "Mr. Mega Man".  We really need more friendly killer battle robots in the world.   

Ring Man
(Mega Man 4)

Some critics will say that Mega Man 4 is where the original Mega Man line began a creative decline with its unusual Robot Masters, but I say that Ring Man can stand with the best enemy characters in the series.  His Ring Boomerang weapon isn't the most powerful weapon, but he makes up for that with a keen sense of style.  Red is always in season, not to mention the many circles/rings integrated into his design.  The rings on his head are easy one to spot, but also notice his round eyes and "ears", the ring embedded in his chest, and the rings on his wrists.  Then there's that bit of twisted joy I feel when I use the Pharaoh Shot weapon's free energy glitch to dispatch him.  Poor guy; he never had a chance.

Heat Man
(Mega Man 2)

At first glance Heat Man looks a bit goofy thanks to his flip-top lighter design, but his ability to close up into a small box and slam around the room as a flaming projectile can be deadly.  Character artwork produced in recent years gives him a bit of a pout for an expression, as if he's never really angry, just really miffed about something.  His Atomic Fire weapon offered a chargeable weapon before such elements were series mainstays, but it consumes a lot of energy compared to the other weapons of Mega Man 2, so it's not something that most players will attempt to use on a regular basis.  It has its uses though, particularly against Wood Man and his large robotic fire-breathing dogs.

Bubble Man
(Mega Man 2)

Bubble Man doesn't seem to know how to swim very well when we first meet him which is really odd for a robot designed to function underwater.  Instead he travels in short hops.  Still, his Bubble Lead weapon is great for detecting false floors and for extinguishing a certain a Robot Master shaped like a lighter.  Later games finally let him put his built-in swim fins to good use.  It's a shame that we won't be seeing Mega Man 2: Powered Up any time soon.  I'm curious as to how Bubble Man would get around on dry land.  His level's background music stands out from the pack, too.  Oh, and nice swim mask, by the way.

Snake Man
(Mega Man 3)

While his Search Snake weapon doesn't seem to track with what one would expect to find in an offensive attack from an animal known for its poisonous bite, Snake Man lands on my list for one very good reason: his stage's music is my favorite Mega Man level tune of all time.  Seriously.  It's even fantastic as a fan-created remix.  I also like the snake head part of his design.  It creates the illusion that the character is somehow swallowing his own face, and you have to admit that is not something we see everyday in solid game character design.  He also has a habit of hopping instead of slithering like one would expect from a snake.  Dr. Wily must have found a sale on hydraulic hopping legs at some point considering all of the Robot Masters that jump around instead of walk.

Dust Man
(Mega Man 4)

If you thought that Capcom was scraping the bottom of the idea barrel with Robot Masters like Top Man and Skull Man then you're sure to love Dust Man who proved to me that one can take just about any noun and turn it into a viable character.  Vacuum Man or even Suck Man may be a better name for him given that his Dust Crusher weapon involves inhaling nearby things, compacting them into large cubes, and expelling them with great force.  His talents are somewhat of a precursor to Mega Man 7's Junk Man, I suppose.  He's especially weak to the Ring Boomerang weapon, and while I'm usually pretty good at figuring out the rock/scissor/paper weakness system in most Mega Man games, I just can't connect dust to rings in any logical way. 

Stone Man
(Mega Man 5)

Although a little derivative of previous Robot Masters such as Guts Man and Hard Man, Stone Man has a better slam-based gimmick.  While other large characters can shake the ground and stun Mega Man with mighty leaps, poor Stone Man literally falls to pieces on impact and becomes a pile of debris for a quick moment before reconstituting himself for another attack.  His Power Stone weapon isn't that effective on Mega Man 5's quick-moving enemies, but it's difficult to top smashing oneself to bits and then standing up without a scratch for a counterattack.  There's just no beating that from a style point of view.