For weapons I've fired, it'd have to be a competition between the Makarov and the PSM. Firing those is like trying to stick your index finger in a vice grip and trying to somehow pull it apart. And even if that works, start praying for God to make that bullet fly to the target area, since otherwise it's not going to do anything.
If one is looking for the universally most hated weapon, then the Chauchat is probably a fair bet. It really is one of the worst ...things ever created.
A lot of American weapons are also somewhat unpleasant to me. Particularly the M16. I'm not saying they're bad (apart from the jamming...), but as many as I've held, they all feel unnatural to my poor European hands. Completely a personal thing, you understand.
I'm happy with weapons of the Fatherland, thank you. The only thing that comes to mind that was really, really bad was the PzB 39 upon first issue.