My weapon of choice is the .52 Gal Deco. It's not the fastest shooter, but it Splats opponents quickly, and I can out-do a lot of Aero Spray users in terms of turf. Unfortunately, it's one of the few guns that is at a disadvantage if a team member is using a defense build. For this reason I prefer to attack by sneaking. It gives them less time to react, and more time for me to Splat them. Its seekers are great for a few things: sending it over a ramp from where opponents can't see it to splat them, targeting opponents directly, escaping, and infiltrating. It also has Inkstrike, which is basically my favorite thing ever.
Aside from that, some general advice would be:
Don't Super Jump to a teammate that looks like they're close to enemy ink. It's very easy for enemies to sneak up on them and Splat them, and then Splat you. Try to Super Jump to someone who's further behind, and then swim the remaining distance while looking for signs of the enemy. On that note, listen for the beeping that comes with Super Jumping. If an enemy tries to ambush your teammate, you can use this to sneak up on them. Alternatively, you can ambush Super Jumping enemies as well.
Don't look down at the ground when using a shooter. You cover more ground if you aim the guns up, as they have more range than it may look like. You can test them in the weapons testing area and shoot at varying angles, to see what works best for your gun of choice. That and looking down makes people less aware of incoming enemies or dangerous traps.
For rollers, I generally find that newer users forget to shake it to Splat enemies and instead try to run me over even when I'm facing them. Rolling over people is great and all, but works best if you sneak up on someone and do it. If you're facing off against someone, it's generally better to shake it. They could Splat you in the time it takes for you to reach them. The Dynamo rollers are especially terrifying when shaken. I hate facing off against them in narrow maps because they're incredibly hard to escape from.
If you're looking to make a good set of gear look at brands, as well as the abilities. If you love Inkstrike like I do, the special duration up ability is pretty useless. The brands are important for rerolls if you got sub abilities you didn't like on your small slots. They go something like this:
Splatoon Brands said:
Firefin - 5x chance for Ink Saver (Sub), 1/2 chance for Ink Recovery Sub
Forge - 5x chance for Special Duration Up, 1/2 chance for Ink Saver (Sub)
Inkline - 5x Defense Up, 1/2 Damage Up
Krak-On - 5x Swim Speed Up, 1/2 Defense Up
Rockenberg - 5x Run Speed Up, 1/2 Swim Speed Up
Skalop - 5x Quick Respawn, 1/2 Special Saver
Splash Mob - 5x Ink Saver (Main), 1/2 Run Speed Up
Squidforce - 5x Damage Up, 1/2 Ink Saver Main
Takoroka - 5x Special Charge Up, 1/2 Special Duration Up
Tentatek - 5x Ink Recovery Up, 1/2 Quick Super Jump
Zekko - 5x Special Saver, 1/2 Special Charge Up
Zink - 5x Quick Super Jump, 1/2 Quick Respawn
You can use abilities to compensate for negatives (like Ink Savers on a weapon that uses a lot of ink), or to maximize on a positive, like Special Savers and Special Charge Ups for a special you really like on a weapon, or to suit a certain play style. Because I like to sneak around and escape if I'm stuck in enemy ink, Ninja Squid and Ink Resistance are my great buddies. Both are restricted to main slots only, though, and neither can be stacked. You just need to define what your play style is and what you need based on that.
EDIT: Oops, I guess I should say something about ranked, too!
Stand near, but not on the Splat Zones whenever possible. The Splat Zones are always, always a target, and if an opponent Splats you in one, you just helped them get closer to controlling it.
Do not be afraid of standing on the Tower in Tower Control if you need to. Use the spike at the center as defense when needed. If facing a charger, move around quickly and shoot at the tower's floor, as they will restrict your mobility and eventually Splat you if you don't. Watch out for Splash walls enemies put in the way, as the tower can pass through them, but you can't. If you have mines as a sub-weapon, these are actually great to place on the tower itself. As soon as an enemy is on there, it will scare them away.
Do not be afraid of holding the Rainmaker. It's scary having you be the focus of the enemy team, but if you have someone with you, you can stand just behind them and provide support with the huge inkzooka-like blasts. If you need to retreat a bit to charge and take care of someone, that's fine, but make sure you're still facing them a bit so you don't lose track of them.
Also, never ever stand between the Rainmaker's shield in the following situations:
If the shield is mostly covered in the opposing's team ink, as you get insta-Splatted when the shield pops.
If you are between the shield and the wall. The shield deals damage on its own, and it can, and will crush you.
