Both Pokemon are viable and both can fit on competitive teams. Because both are actually really valuable, they can fit across tons of different teams, but each in their own suitable role. Both have different movesets and playstyles and you have to know which one actually fits onto your team.
- Protean - Greninja's type is okay, but it gets Protean which allows it to be any other type in the game provided it uses a move of that type. If you sense a grass move is about to hit you can use Hidden Power [Fire] and resist instead of possibly die from it. It also happens before it attacks so every single attack gets a STAB boost.
- Speed - Greninja has a base 122 Speed, making it one of the fastest Pokemon in the game. It can max out to 376 speed and works great with U-turn.
- Offensive Stats - Greninja has a 95 attack stat and a 103 Special Attack. Though not great on their own, they're pretty special together as U-turn will do you more damage and you can still hit with Hydro Pump.
- Defensive Stats - Greninja has a spread of 72/67/71 defensive stats, meaning it won't be able to take Super Effective hits easily. It can still show survivability with Protean and a skilled player.
Greninja @ Life Orb
EVs: 252 Spd / 4 Atk / 252 SAtk
-Hidden Power [Fire]
This is a set I'm actually using in a Mono Water team, but I'm sure it will still function in OU. There are plenty of other Greninja sets including completely physical or special and even sets utilizing Spikes.
- Typing - Azumarill got Fairy added so it actually became better at handling other types. It can kill dragons and beat Dark and Fighting. It now also can absorb Dragon attacks and handle a strong Knock Off with relative ease.
- Huge Power - Huge Power can be used with choice Band to give Azumarill over 600 Attack. Only certain Mega Pokemon rival its power.
- Aqua Jet - Because Azumarill is so slow, it almost needs this move. Aqua Jet is STAB and it pretty much allows you to outspeed whatever else.
- Bulk - The 100 base HP isn't bad actually and it allows Azumarill to tank hits and be able to put that together with Belly Drum to become a complete killing machine.
- Underwhelming Stats - Without Huge Power, this thing is garbage. The only bright spot would be that 100 base HP as it only 420 (blaze) BST.
Azumarill @ Choice Band
Ability: Huge Power
EVs: 200 HP / 252 Atk / 56 Spd
This is the band set that starts killing as soon as it hits the field. Azumarill can also use SubPunch and even a Belly Drum set and they are all viable.