I wouldn't say that I can nail down a favorite, (split between Metal man, Bombman, Flashman, Gemini man, Fireman) but I am not as fond of many of the later entries in the series except for 9 (as I have not had any problems with the games in which Inafune did most if not all of the character designs 1, 2, 3, 9)
I felt the designs of the Robot monsters in 1, 2 and 3 were home runs.
-I go into further detail regarding certain robot masters in my spoiler, feel free to skip it, but there is also some props to one particular game in the series, a concept that the series itself should have embraced thereafter.
I find Centaur man stupid as a concept and the name just, I don't know sounds wrong
Yamato man is cool, but naming him after a region (really, thats what Yamato is), come on there has to be something better suited than that, whats next "Jersey shore man" or "the Hamptons man"
Search man is cool, but naming him that is dumb, why not ambush man or something else. The name Search man is dumb, makes me think his specialty should be finding Carmen Sandiego and Waldo. Even "Grassy Knoll man" would be better
and although they kinda managed to pull it off don't get me started with Sheep man as a concept.
The arts style and concepts for MM(pc) sucked, and on top of that Dyna man..really.. a nuclear powered weapon is a bit much
the Megaman pc 3 tile was leaps and bounds better, and better captured the style of the nes series though still had its problems
Sharkman, (not all for naming after and stylizing after animals (though they seems to pull it off in the x series, because the animal itself isn't the power)
and Bit man was a weak of a concept to begin with...
Now we come to Compass man, perhaps the biggest fail of all, that would be a perfect name if his power was pointing north.
I loved how they used our solar system as a theme for Megaman V (not 5, the one for gameboy, that was a genius idea IMO)
Would have like to see more Megaman games built a round a central theme like what was done in 5. Perhaps robot masters built around the Zodiac and have Gemini man return for that, or have a game with masters built around precious and no precious stones: Diamond, Emerald, Ruby, Onyx, Sapphire etc
Or Elements from the periodic table, there are so many great ideas to choose from, such as: Gold, Silver, Titanium, Helium, Platinum, Lead, Hydrogen, Helium, Nitrogen, Plutonium, Mercury (liquid metal, completely different that V's stardroid)....