I would actually say that Lanturn is the best bulky Water-type in RU at the moment. With the rise of Pursuit trappers in Escavalier, Drapion, and Spiritomb, and the fact that Slowking can't stop some key special attackers (such as Nidoqueen and Hurricane Moltres), Slowking just doesn't quite cut it anymore. Lanturn's Water / Electric typing and Volt Absorb is very, very beneficial in this metagame. It's basically the only Pokemon that can reliably check Moltres, which is reason enough for any team to use it considering how difficult Moltres is to stop. It's not like Lanturn only checks Lanturn and nothing else either; it still checks a bunch of other stuff too. Magmortar, Magneton, Manectric, Omastar, Rotom, and Typhlosion are also checked easily by Lanturn in addition to Moltres. It also provides valuable cleric support for stall teams, and with the RestTalk set that DittoCrow posted it'll usually stick around long enough to keep curing your team of status and keep checking those threats.
I think where Lanturn really excels is on balance teams though. Volt Switch is extremely useful to keep momentum in your team's favor, and Lanturn's ability to check all of those Pokemon who could tear through balance teams makes it an excellent utility counter as well. It works really well with Moltres or Entei on these teams, since Lanturn tends to lure out Grass-types, and those two can easily rip huge holes once they get into the battle. Lanturn can also lure in opposing Lanturn though haha, so using an offensive Grass-type like Tangrowth or Sceptile works nicely to get rid of the opponent's Lanturn; they also complete a F/W/G core in tandem with Lanturn and the Fire-type of your choice. Defensive Lanturn is really good at the moment guys; use it!
Lanturn isn't just a defensive Pokemon however! A more offensively oriented Lanturn still works very well, even though it does make it so that Lanturn can't really check Moltres and such as consistently. Still, Water / Electric STAB attacks let Lanturn hit surprisingly hard, and it still has a powerful STAB Volt Switch to keep momentum. I've personally had a lot of fun with Expert Belt Lanturn; it might seem really weak, but it's fun since it bluffs Choice Specs really well, and Lanturn's great coverage lets it hit the Pokemon who think they can switch in on Volt Switch very hard.