this is on 3ds, so quality warning right there
lets start off the bat with bulky water types.
good with drizzle ability and hence rain teams, but has crippling weakness against electric attacks. edit: pelliper does not have drizzle, yeah...kinda big deal that...
i am surprised that golduck is as common as it is, works well besides pelliper, and that was where I saw one last. edit: in spite of seeing golduck in cybertrons road to ranked, it is not actually a good Pokemon in competitive, kind of like je juun parks pacharisu (a battle I will never forget)
kingdra has the epic typing of water dragon, and if set up, can become an unstoppable menace, I would say kingdra is kind of famous for rain teams IMO. edit: sumwun just informed me that kingdra is outclassed, I had one of those being trained in poke pelago, so I am kind of dissapointed that he did not tell me earlier.
suicune is my favourite legendary Pokemon, double points if you name it reign. edit: suicune still rules, hah!
probably misspelled the name, so don't rely on the link there, I added this one because it is a gen 6 Pokemon, though it does come with a few weaknesses, it can hit pretty hard. edit: I literally added barbuckle because my multi battle partner in sun uses it (does anyone know how to get another battle partner in sun?) so yeah...what sumwun said.
i doubt it's the same with XY, but gen 7 has cloyster dominating the competitive Pokemon field, though, again, not gonna be the same in gen 6.
and that is the best job I can do with a 3ds and 20 minutes of my life, if it aint good enough just blame my phone for spontaneously deciding not to charge ever again. I typed the name of the Pokemon in the links manually (again, 3ds reasons) so the links might not work.
edit: seems I missed some pretty big players, as pointed out by sumwun, including the likes of slowbro which I think has a mega in XY. slowbro is very common in my experience, and very well known for its bulk, and by memory, isn't to shabby offencively either, so definately consider that dopey destroyer.
sumwun also mentioned starmie which by memory makes a good sweeper, and is a Pokemon I do like, though, certainly has less bulk than the likes of slowbro...obviously.
finally, I can't believe I forgot the one and only rotom wash, a very common water type indeed, something I seem to always see in things like championship battles, with...well, some solid pros, I never used one, so definately check the ever smarter then me smogon.
hope I helped (sumwun would've (that's a pun you see))