Ooooh, fancy. I have a somewhat wide range of mains, so I'll try to list them in a somewhat organized fashion.
Sloshers: I like Sloshers. A lot. I'm pretty adept at utilizing their arcing capabilities too. I prefer the standard over Tri, but I think Tri is more competitively viable right now if only because of its Bubbler. I'm pretty decent at Burst-Canceling with the vanilla Slosher, though. Once the Slosher Deco comes out, you can have no doubt that I'll be maining that thing like crazy, since its kit is practically made for competitive. But until then, I probably won't use Sloshers much in scrimmages.
Octobrush: The Octobrush can be awkward for me at times, but I'm becoming more and more experienced with it. When I play it, I tend to lean toward a support/assassin role, setting up Squid Beakons at key locations, usually fairly safe ones, and picking off any unsuspecting targets. And then, of course, there's the Kraken, which I don't think I need to explain.
Chargers: Out of all chargers, I'm most experienced with the vanilla E-Scope. However, I've been recently practicing using the scopeless version of it, as well as other chargers, such as the Hero Charger Replica and Kelp Splat Charger. When using these, I play the sniper role that almost any charger does, being the team's "eyes in the sky" and picking off targets from afar. With the vanilla E-Liter, I'm adept at using Burst Bombs in close combat. With the Hero Charger, I can use the Splat Bomb Rush to claim large amounts of turf or hold choke points on the map. I'm also pretty good at placing Sprinklers in clever locations with the Kelp Splat Charger.
Aerosprays: While not the most reliable in combat, the Aerospray RG was my first ever main in this game, and so I've learned its weaknesses quite well and I do my best to avoid letting others exploit those weaknesses. I guess the role I play with these things is the support/scout role? With the Aerosprays' quick turf coverage and maneuverability, I can move quickly throughout the map, cover ground for the team, and take out unsuspecting opponents. In Splat Zones and Tower Control, I tend to use the Aerospray RG over the MG. With its Inkstrikes, it can turn SZ matches around quickly, and its Ink Mines can easily catch opponents off-guard in TC. In Rainmaker, I prefer the Aerospray MG. Its Seekers allow me to get to mid quickly, and they are also very difficult for the RM carrier to avoid. Finally, its Inkzooka can make up for its horrible lack of range.
Carbon Roller: Basically just copy/paste what I said about the Aerosprays here :p Though I can also Burst-Cancel with this, if that helps. I usually use this weapon in Rainmaker, as the quick path it creates can help the team to push the RM quickly, and its insta-kill capability can help it fend off opponents who get too close.
Octoshot Replica: This my best offensive role weapon. It's just a good weapon all around. I tend to do well in firefights with it. I can easily hold choke points with its Suction Bombs, and the Inkzooka can be used to pick off enemy snipers.
P.S. This took a long time to write XD And for goodness' sake, Aqua, capitalize the title of this thread!