i naver have too much of a problem with bosses but 2 come to mind of being difficult 
the bosses of double dragon 3  
remember in this game u dont get lives u just get alies and i think i remember 4 chooseable chars later in the game  and when that char died it was dead  and each char had its own ablity 
remember the last level with the bottomless pits u had to dash over?  i considered them to be a boss in them self because ur movement had to be 100% flawless. you never had to jump in the game untill the last level. (nice time to pratice) 
if any of ur chars were dead at this point u lose because u cant win against the boss with only one charr. best thing to do was just turn off the system  toss it back on the shelf and start playin mario bros before u hurt urself or take it out on the dog 
and if u managed to make it across the gaps (which were in the beginning of the stage) u had to complete the stage and kill this crazy kickboxing mummified ninja badass from the darkside that had like 99999999+ hp  
he would begin pounding on you to the point that u couldent get up to defend urself.   i couldent even cheat to beat this dude because hp dosent matter if hes got u in a corner beating the hell out of you and u cant move. 
i did beat him one time in a 2 player game, but i think in 2 players there are 2 mummys not just one (to make it even) and u just had to tag team