Wow, this is a super hard thread to answer - I don't think I've got a strong dislike for any firearm, they're all fascinating
If I have to pick one, it would be the STEN. But I understand the lack of aesthetics and I'm not a big fan of left sided feeding - but back then all they needed was a tube with springs to direct a projectile toward an enemy. Still a pretty cool and skeletal looking gun though.