I don't know about worst, though I definitely need to use more weapons, and more often. Recently I've been practicing with carbon, and noticed the need to work on aiming compared to the brushes I am more comfortable with. And I did not appreciate until now how it has even less range than the Octobrush, which makes it unsurprising that, like with the brushes, I found myself ambushing a lot.
I'd say I'm worst at Blasters and Splatlings, although I haven't given either much time. I'm not the best with a charger, but I have been practicing some lately.
Splatling. I usually use rollers, am neutral about shooters, getting used to using chargers on certain maps. I've actually really rarely used a slosher though, so maybe I could be bad at that, but I don't really know, and I doubt it's harder than a splatling. They just feel SO limiting.
I suck at the sniper charging type weapons. I get bored waiting around and want to swim closer to the enemy where the enemy is to shot them like I would with a regular shooter. Then I might get one with my subweapon but quickly become toast.
Roller for me. I tend to try too hard to emulate the Japanese players and I'm not very adept at pressurization by throwing ink and I usually get splatted the most with this class.
Shooters. I have no idea why I can't get a handle on them. The others I'm not too bad at, charger I do find pretty difficult though. Would really like to at least master one of those, charged would be my top choice
Out of the list above, probably sloshers are my worst. I just simply don't use them enough and I find that they run out of ink too quickly for my liking.
But my absolute worst weapons are the Nozzles (seriously who thought of them?!?!) and Dynamo Rollers. I don't think I'll ever get used to those types of weapons