A classic choice would have to be the Cyberdemon from Doom, for all the right reasons. Big, strong and carries a massive rocket launcher (and a proper rocket launcher from the Doom days at that - even if you somehow survived would leave your screen red and fling you up against a wall, not these pansy peashooters you get nowadays). Stays true to my preferred boss design i.e. no gimmicks required to kill it, just a ton of firepower.
I was already to respond with my standard rant of how I hate bosses, and the arbitrary gimmicky play you need to beat them, till you reminded me that all bosses aren't like that.
Slightly off the wall choice but Shivan Juggernaut from Freespace 2, when you distill it down to the base properties, has plenty of characteristics of a good boss i.e it is completing ****ing badass while retaining an air of mystery, and instilling fear in the player - one wrong move and that beam cannon could seriously ruin your day.
Golden Axe and Serious Sam stand out.