Wait wasn't there another thread I replied too that was like this a while ago? Eh... whatever my answer did change so the answer is pretty outdated. Although might as well name what my last most hated weapon was.
Before I would of said Jr. because of the weapon tendency to play far away, and spam bombs. While I'm still not a fan of the whole sit back, and spam subs, and specials playstyle I gotten more used to the weapon playstyle, and now I can say it doesn't bother me as much.
Now despite me saying that I don't like fighting weapons who just sit, back and spam my least favorite weapon to play against is the complete opposite of that, and that is inkbrush
Inkbrush whole deal is to use its speed to be a general annoyance to the enemy team, run behind everybody, and then attack when everybody guard is down. Now yes my main roller also has this playstyle to an extent, but I feel that with roller you can't just run in you need your team to paint for you so you can get the jump on people. With inkbrush you can just go anywhere you want, and be hard to contest.
It also doesn't help that 1 of the 3 weapons I play has a pretty rough time against inkbrush that being s-blast due to not being able to keep up with it. Lastly the most annoying thing is the noueavu kit the vanilla one is annoying, but the noueavu kit is next level.
Trying to chase the weapon down is now an even bigger pain since you get hit with a bunch of mines, and the rare time you do catch up to them you get instantly stamped.
Once again my mains don't help me here against a stamp, Splat Roller can maybe trade with its roll mode, but S-blast, and Painbrush just dies when a stamp rushes them.
Honerable mention to Neo Sploosh it also fits the bill of being an annoying rat weapon that gets behind you, place beakons, and forces you to go out of your way to deal with it, but Neo Sploosh players are 1 in a million, and good Neo Sploosh players are 1 in a billion so I don't care as much.