These 2 are both in the A tier on the recently updated OU viability rankings, so this is going to be quite hard.
It's pretty hard trying to compare both of them, since they technically do different things. Slowbro has an easier time switching into physical Attackers, while Slowking is better suited to coming in on Special ones.
I'd say go with Slowbro. It isn't as weak to U-turn and Knock Off, and it doesn't fold to Earthquake from the king of OU, Landorus-Therian. Slowking is supposed to check defensive Pokemon, however it takes an unnecessarily large amount of damage from a Toxapex Knock Off. It also struggles with Ferrothorn, since it can use Power Whip. Tornadus-Therian, a Pokemon Slowking is supposed to check, uses Knock Off on pretty much all sets, something it doesn't appreciate. Grassy Glide Rillaboom destroys it too. Landorus-Therian, Garchomp, or Hippowdon can block a Zapdos's Volt Switch for Slowbro. Slowking is still useful though, since it can switch into Nidoking.
Toxapex is a better overall defensive Water type since it has more well-rounded defenses. It isn't weak to Knock Off, resists U-turn, isn't weak to Grassy Glide or Power Whip. Its only major downsides is its lack of Future Sight to threaten itself, and its inability to come in on Earthquake.
Clefable is a Teleporter that resists both Knock Off and U-turn, and can hold Leftovers. It has a better defensive typing and also packs Wish. However, it lacks Scald, Future Sight, and Regenerator (although Wish kind of makes up for the loss of Regenerator)
Blissey is yet another Pokemon that can Teleport. It trades Regenerator, Future Sight, and Scald for the highest HP stat in the game, great Special Defense, Seismic Toss, and Stealth Rock.