Alright to actually put a meaningful topic of discussion, I've been putting rain through a lot of the changes to USUM, and I've been using mons that specifically benefit from the rain through typing and abilities.
I think Azumaril gaining Knock Off can be a huge surprise if Ferrothorn or Toxipex comes in expecting to wall a Play rough or a Liquidation/Aqua Jet, and getting their leftovers/rocky helmet smacked.
Toxicroak got super buffed, specifically with Gunk Shot, getting that stab 120 poison, also gaining drain punch and knock off, and I've even played with Super Fang as an interesting twist. Again because I throw out my frog and they try and switch out to a Ferrothorn or a Toxipex or some other physical tank to take the hit, and suddenly half health and are in OH/2H KO from another move on Toxicroak, or something else.
I think people are really down on Araquanid, I think it fills a nice niche in OU, and I think it's a solid C+ student, and that's ok, that is still passing the class. I personally hate using Smergle, and think you can do a few more things with an Araquanid lead. Lunge, Liqudation is an interesting combo along with Sticky Web because it means any switch in Pokemon could come in with 2 stats lowered. Leach Life keeps it healthy forever, and also I've seen a set where people experiment with bug bite since depending on the team you could see a few berry holders.
This is just my blanket statement for all physical water types, run liquidation. Effects of the two moves aside, just the 5 more base power should be more than enough reason to run it, like Swampert can 2HKO Spedef Skarm with rocks, and the chance to lower defensive stats I think is more valuable than a chance to flinch. So yeah I think the only physical water type you might not run liqudation on is Crawdaunt because it fights an even more damaging ability for some accuracy or priority.
Some of the cool defogers that Rain now has, obviously Zapdos and Thundurus, and personally I like Zapdos more since i think it does more things, but priority defog is pretty cool. Also I think we could see maybe some use of a Physical Thundurus, or a mix set, base 115 atk is nothing to sneeze at, and getting knock off and thunder punch and Superpower there is a world where this is an option. Thundurus-T is mostly, it can do defog also which is still a yay i suppose. Rotom W is a huge winner of Defog, but that has already been talked about, but i think it just makes Rotom W an even more solid mon, defog and a status move is great. And just since people are talking about it, even though it's not really on the viability list, Empoleon with defog is nifty and I think there is an actual conversation around it.
Quagsire and Gastrodon get Earthpower because I know some people really don't like running a nature that weakens EQ, also I think Icy Wind is something cute they both get that can put people in an awkward spot if the switch out and suddenly that pokemon is crippled a bit.
You can now run a physical Manaphy with liquidation and knock off and Z bounce..... because idk I won 3/5 games, and that is the whole of my argument, no one expects it so if you like doing these things, it's a thing.
Kingdra getting Iron head is actually huge, it finally gives it a fairy type coverage and I think makes the DD physical Kingdra, along with the bit about Liquidation I think you have an actual viable physical set for Kingdra, I don't think it trumps the special version but I really do think Iron Head is huge on Kingdra, also Z Bounce get's rid of Spdef M Venu, Keldeo, Tangrowth, and a few others.
And just to round out and mention other things I've had a lot of fun with that i think people should try out on their rain teams.
- Seismitoad got access to Knock Off and rocks and can sorta act like a Diet M-Swamper, if you wanted to use that mega spot on something else.
- If you really really wanted access to Heal Bell, and don't feel like running Chansey or Blissey, Vaporeon is a totally viable option as a water cleric.
- Hawlucha got a punch of new stuff for coverage if you wanted it, it's still the same ol scary thing, but you get Thunder/Fire/Drain Punch, Zen Headbutt, Defog and Tailwind if you wanted a bit of self support, and I think the biggest it get's Iron head to deal with fairy types
I didn't go over every single thing you can run on a rain team, because I haven't used everything, but these were the things that really stood out to me when I was testing things, or just things I found kinda cool and unique, and maybe some things that are just outside the meta but are viable.
**note** I know Aqua tail is a move tutor but I don't trust 90% accuracy, because I think every single person here knows that 90% chance to hit, really means, going to miss 90% of the time.