For you to win any ranked games, you need to know how to flank. You need take your opponents by surprise and splat them so it will be safe for either your or your teammate to get the rainmaker or tower. You need to know your maps and good places for you to get behind your opponent and flank. You can press y when selecting ranked mode to do a recon on the maps and watch youtube videos on different paths you can take. Once you successfully flank, go ahead of the team and try to create a path for your team to win. In rainmaker, you should always try to cover as much ink as you can leading to the goal while in tower control, you should focus on taking your opponents by surprise and if you can't, try to be the last man standing in a fire fight. Don't just blindly rush straight into your opponents face, instead try to take higher ground and splat them from there. Or, if your opponents are on those metal bridges, try splat them from underneath.
Also, you should have accompanying weapons and gear to go along the maps. For somewhat narrow or one way maps in rainmaker mode, you should get a long ranged weapon like the splattershot pro or a splattling and a seeker. For your special, you should get an inkzooka as it can clear and make a path really fast. For tower control mode, always get a weapon with sticky bombs, as they can come in handy for getting your opponents off the tower. For your special, you should get an inkstrike since it will be easy to aim since the tower is slowly moving towards the goal. Now, ultimately, the best weapon for you on any map is the weapon you are most used to. Of course you should try other weapons and see whether you have an affinity for it, or are willing to take the time to learn the weapon, but you should do so in normal battles and not ranked, of course.
Now for gear, it depends on what weapon you main. For me, I use the standard splattershot and I try to have a speed based build. Definitely max out swim speed. Opening gambit is useful for early game flanks which can definitely earn you a fast win if you make a good play. Walking speed is optional. It's really only useful for strafing. If you have a short ranged weapon, don't get walking speed, instead get something else. And of course, use shoes that have ink resistance. It can really get you out of tight situations.
For a roller, its best you max out your defense. I don't know too much about what other abilities you should get on it though. And last, but not least, for sniping, you should get support abilities, like special charge up, bomb range up, bomb sniffer, special duration up, etc. You won't be needing any ink savers since you're sniping.
TL;DR: Know your maps and best places for flanking (best way is through youtube videos and recon), choose a weapon that will be advantageous for the ranked maps, but also a weapon your familiar with, and choose sub and special weapons to accompany them. And likewise for your gear and abilities.
