Samurott. Wailord has great HP but rubbish defences and speed. Samurott has the benefit of being able to carry physical or special sets, so your opponent is never totally sure what set you're running before you make a move.
Samurott has base 95/ 85/ 70 defences, which isn't shabby, and 100/ 108 offences (which are also higher than Wailord's). I think Wailord really only has the benefit of Water Spout (which it's not particularly fast enough to utilise before its HP decreasing) and Pressure.
In terms of offensive types, however, Samurott is considered slow at base 70 speed, but not slow enough like Escavalier to be able to fully abuse Trick Room.
And while I did say that Samurott can run either set, it does have a shallow movepool. It's rarely a good sign when a Pokemon has to rely on HP for coverage. If this isn't NU, I think you you have better Water type choices to consider, like Feraligatr, Kabutops, Quagsire, Suicune, Kingdra, Keldeo etc.
