Anything with the Kraken is great, since you can hunt down and destroy the rainmaker holder. Ranged weapons are great, so if you don't like chargers, you might want to look into the .96 Gal Deco (the Splash Wall helps force a shootout rather than a chase) or the Custom Jet Squelcher (Burst Bombs cut off avenues of movement, especially up walls).
The Splatling is great with the Splash Wall, good range and power, it's a shame it can't take much advantage of the Inkstrike - at best, you can use it to hit where the enemy is hunkered down at, or to get a sure shield break. The Krak-On Splat Roller is good for the larger maps like Moray Towers, so you can take full advantage of the beacons, since most people don't pay them much mind in Rainmaker - otherwise, the regular Splat Roller or Gold Dynamo are the preferred rollers.
A good way to shift matches in your favor is use the shield as bait, and circle around your enemies while they are focused on breaking it. You can get one or two surprise splats this way, giving your team some time to grab the Rainmaker and push it forward a bit more before the enemies return.
Keep the pressure on the Rainmaker holder if you don't have it - take advantage of strategies that wouldn't normally work due to enemy mobility, like bombs or the Killer Wail or others, to splat the holder. Grab the Rainmaker if there's no one else grabbing it, but if you're bad at carrying it wait a few seconds to see if a nearby ally will do it instead. If you do hold it, play conservatively and stick near your allies - at the very least, they'll be a distraction for your opponents, at best they'll give you a long path and some covering fire. As a supporter to the Rainmaker, when making a path don't stray too far from him, and keep in mind his limited mobility. The most direct route isn't always the safest, so if the enemy team is concentrated around one route, try to lead the holder to another.