My least favorite Robot Master is actually Sonic Man. (Yes, that is a legit robot master made by Capcom)
And for numerous reasons:
1. Mega Man DOS sucks. Let us get that out of the way. It's the CD-I Zelda of the Mega Man franchise. Even something like X7 is more appealing. You want to know how bad it is? 3 Robot Masters, ugly graphics, and Mega Man doesn't even change color when switching weapons.
2. Sonic Man has no theme, just like the other DOS Robot Masters. In fact, the game has no music.
3. He is a water robot for some reason. (Sonic the Hedgehog hates water so this feels wrong to me)
4. His purple flippers look ridiculous, they couldn't have gone with a neutral color?
5. His color scheme is just unappealing in general, green and purple feel disgusting to look at when the Robot Master itself is already kind of ugly.
6. That name. That god damn name, it just makes me cringe. It's not the Sonic part that makes me cringe, it's the fact that Sonic Man is a legit Robot Master made by Capcom, which makes it feel awkward. Also, Mega Man DOS was a 1990 game. Sonic the Hedgehog was a 1991 game. So this abomination is actually older.
(Off Topic: I remember playing a fan-made Mega Man game called Fega Man. Instead of Wily taking over, it is actually Adolf Hitler, and Mega Man is one of the robots that is reprogrammed for world domination. You play as a former Nazi by the name of Hermann Fegelein who was actually loyal to Hitler in real life, despite rebelling against him in the game. The game is heavily based off of the movie "Downfall", and it's a pretty weird game. Seeing Mega Man doing the "Sieg Hiel" pose was the icing on the cake when it came to weirdness though.)