While there are a lot of bosses I simply can't beat, I reserve my deep hatred for Egg Emperor. The ONLY Sonic Boss I can't beat without my glasses but can (barely) beat with them.
Although if I ever get that damn seventh chaos emerald, Metal Madness may join the ranks.
I actually agree with that a fair bit.
Egg Emperor is a pain. Your two methods of dying are a) falling off by failing a jump [aka him hitting you during one] and b) getting pulverized with no rings during the circle arena. [aka getting too close to him]
You can avoid meeting the fate of (a) 99% of the time by timing your jump, and avoiding (b) usually involves smashing the cannons, (free rings), team blasting, and then just thunder shoot etc.
Even so, I still find him a pain to beat.
Now with the Chaos Emeralds, I found the even-numbered ones easy (also the first), but 3, 5, and 7 to be hard. (Interestingly, I found 3 to be by far the hardest, and there was much rejoicing when I finally got it) After I did get it though, I did some dedicated SS attacking and got the other two.
I won't spoiler Metal Madness/Overlord, but I will say he's actually quite easy, once you figure out what you are meant to do.