For a while the top threat on my radar was the .96 Gal Deco. That was at a time when I insisted on using shorter-ranged weapons, like the Krak-On Roller and N-ZAP 85, thus I didn't really have a way to deal with the gun's impressive range and firepower. Nor could I ever figure out a way to get past their damn Splash Walls (which I maintain are too perfect a fit for that kind of gun). So inevitably I'd lose just about every match that had one on the enemy team.
Now it is still a threat, if only because they're usually skilled at picking off weakened squiddos from afar. But my new public enemy #1 is the Zink Mini Splatling. The people that can use them are aggravatingly good when it comes to raising hell, and the Bubbler special ensures that even if you can get the jump on them, you still won't be able to do a thing to them. They're a little easier to combat due to their slightly lesser range, but if they land that Disruptor on you... well, you'd better hope you can Super Jump away in time. I don't think it's quite broken, but it's certainly a doozy to fight off.
Also there's the Dynamo Roller, but I agree with the school of thought that says it's only a problem because of lag.