Gonna drop a neat core I've been using with a bunch of success: The Dogsquad
CM+Roar Suicune+SubCM Raikou
This is kind of my take on an old core used to a ton of success by players such as the legendary Mariano Dragon, the conjunction of Suicune and Raikou to pull off a sweep. I've never been a fan of CroCune as residual damage tends to mess with it while it's so damn passive......the new ResTalk mechanics suck too. CM+Roar Offensive Cune, on the other hand, is a beast that plays surprisingly similarly to the Offensive Starmie that everyone's raving about (in casually switching in and blasting away full force), with the ability to boost up as well. SubCM Raikou, backs this thing up by using its own bulk to waltz in on some of Suicune's checks such as Zapdos and threaten those, making an extremely lethal pair. The kicker though is the set combination: CMRoar sets up a +6 on Calm Mind Blissey, before using Roar to negate Blissey's boosts and allow you to sweep: this also bypasses other CMers like Celebi, Jirachi and opposing CroCune. Raikou, on the other hand, uses a Sub to block bulky Blissey's status and set up its Calm Minds before TBolt hammering it into the ground, meaning that theoretically the core beats Blissey on its own :)
It's not perfect though, as physical offense usually tends to mess it up: Tyranitar's sand compromises their Leftovers, while Snorlax and Metagross can absorb their hits and threaten to blow them up: Aero also threatens to punch through them in the lategame. Jolteon is a pain as it outruns both and threatens to roar them out or wear them down with Toxic+attacks (it's not coming in on Cune though). Dugtrio stops Raikou straight up and can also pick off Suicune if it's weakened. Spikes also really suck for both mons though Skarm doesn't set up on either of them (if it tries then it dies). Lastly if you're running HP Grass Kou (which is inferior to HP Ice in this core but w/e), Bulky Celebi walls you while CB Flygon becomes really hard to handle. Good partners to handle the majority of these include Metagross, Wish Jirachi, and Dugtrio (which also tend to hammer Bliss which you should still pack a back-up plan for). Snorlax handles opposing special walls (Blissey, Snorlax, Regice which can be a pain) while luring Metagross, Tyranitar, Skarmory and even opposing Snorlax (assuming you have EQ/Boom along with Curse), and sponges against CMers that set-up (as neither dog should be switching in on them). Ludicolo is a good check to Blissey as well as Metagross and removes TTar's sand with Rain Dance, while also helping them recover health with Leech Seed. Flygon provides a Rock resist, eats Celebi and dissuades opposing Flygon/Dugtrio from locking into EQ. Porygon2 covers Raikou against Dugtrio while also warding off Salamence, Flygon and Aerodactyl for Cune. Anything can work so long as you make sure it supports the core (which can technically beat all the stuff mentioned in the right scenario).
Here's a list of all the partners I've used so you don't have to sift through my rambling (though do read it so you understand the rationale):
Jolly CB Flygon
Adamant Dugtrio
Bulky Ludicolo
WishCM/Wish+BSlam Jirachi
CB/Offensive Metagross
Curse+Boom Snorlax
Explosion Gengar
Bulky Porygon2
CB Tyranitar
And here's the Threatlist:
Special Walls: Blissey/Snorlax/Regice/Metagross
Aerodactyl/Jolteon w Spikes
Dugtrio
Bulky Celebi: Even with Ice moves on both mons this can be hard to handle, pack something to remove it or you'll be sorry.
Physical Offense/Offensive TSS Teams
Sets:
Suicune @ Leftovers
EVs: Pretty variable: Run 200+ SpA for the damage output, plus some speed to creep on TTar, Meta, Mag, Pert etc., with the rest in HP for pivoting+bulk
Modest Nature (Timid works if you want to outrun some things like Hera and AdaMence but it's an inferior option overall, especially seeing as how if they're running Max speed then your investment's worthless)
-Hydro Pump
-Ice Beam
-Roar
-Calm Mind
Raikou @ Leftovers
EVs: 208 HP / 80 SpA / 44 SpD / 176 Spd - Does a bunch of fun things like ensure you can take a Pert EQ from full, meaning you can set up on it if you'd like. The SpDef means Zapdos and Gengar can't break your Substitute making both of them free set-up, speed outruns Adamant Dugtrio as well as +Spd base 100s and most Gengar (mainly the status variants as offensive variants are doing jack to you while running scared from TBolt).
Timid Nature
-Thunderbolt
-Hidden Power Ice (Removed HP Grass as you should have a partner that can scare Pert out, while Bulky Celebi is a lot harder to handle, especially the Reflect variants)
-Substitute
-Calm Mind