I don't know if this thread is for OU only, but an amazing RU combo I've used to great success is: Steelix/Mantine.
What they have together:
Resist 15 types in tandem, neutral to the other two (Ice/Grass)
Resist each other's weaknesses (Steelix is immune to electric/Mantine is immune to water/ground and resists fighting/fire)
Incredible defenses. Steelix has base 200 defense and Mantine has base 140 special defense. To maximize overall bulky, I use 252 HP evs on both and 252 SpD on Steelix / 252 Def on Mantine, with the corresponding natures.
Great support capabilities. Steelix brings stealth rock to the table and has access to roar. Sturdy allows a makeshift check to set up sweepers if it comes in at full health. Decent attack makes its STAB earthquake hit hard and low speed + STAB let gyro ball do a ton. I personally run Stealth Rock/Roar/Toxic/Gyro Ball, but its set is preference. Mantine, on the other hand, can spread toxic around like no one's business and has amazing longevity with its Rest/Sleep Talk/Scald/Toxic set. If you want to add a bit more offense to it, you can swap Toxic out for HP Flying, but I prefer otherwise.
To summarize, the duo resists 15 types (with 4 immunities), resists (or is immune to) all of each other's weaknesses, has access to hazards, phazing, and toxic stalling, great defenses in tandem and good overall bulk, along with amazing typing individually (but especially in tandem), and decent offensive capabilities.