Slobro with Water Gun/Psychic

Check this post, https://pokemongo.gamepress.gg/Q-A/low-cp-100-slowpoke

I got a similar situation with a 91% slowbro, and basing on gopress tier list i thought he was very good but guys in that post disagree. @Impala said: "Water/Psy is not very usefull type combination. It will still do good damage against anything when attacking but it will always a second choice because you can have a full psy Exeggutor for psy and and full water Vaporeon for Water that will do the job way better.
Not worhting spending dust."

But i think all 100% pokemon are kind of a jewell, keep it and use it for example for prestige your gyms, or in the future maybe niantic releases an option to change moveset!

Depends what you mean by better. He does similar damage with water gun at the same CP as vaporeon but is not as good overall (if the vap has either aqua tail or water jet) for taking down fire types. But really they are pretty easy to deal with using pretty much any old vap (and you most likely have one). So among other likely defenders, very few resist psychic moves and very few do real damage to SloBro, so he is really versatile. That's a brilliant moveset in the long run and if you hatched it, then it will already be at level 20. At around that level, a 1350 to 1400 CP slowbro is a great pokemon for levelling up friendly gyms - enough CP to do real damage to defenders up to CP 2200 or better (if you dodge) without them timing out and just enough HP to take the damage, so great for getting good prestige per heal. Personally, I'd keep it at that level, because where I live Slowpokes aren't that rare, and its a lot less work in the long run to catch a wild one thats more powered up already and with decent IVs. But you won't regret having it on your team as a 'go to' option if you do choose to max it out and keep it there - I have one very similar at 2100.

Slowbro with Water Gun + Psychic is my second most favorite attacker after Vaporeon. It is a very allround attacker. I am the kind of player who dodges literally each and every attack. Water Gun is therefore my most favorite basic attack.

I find it very useful and usually it last pretty long. I would definitely power it up.

If you need a great attacker to your squad, then yes, power it!

I love Slowbro with that moveset, I have one with 84% iv that its part of my attack squad and he never disappoints.