Pokèmon to keep

Depends.
First thing I do is appraise.
If the Pokemon isn't one care for and it's appraisal is weak, I xfer it.

If it's rare but bad, I might keep it just in case. I had a 300 Aerodactyl with optimal moveset but weak IVs as well as cp. Eventually I xfer'd it as I wasn't going to power it up.

Now, moveset is fairly important.
(For gyms and eventually duelling).
I kept a 51 rated Gengar because it's moveset was ideal. 51 still low though.

High CP might not matter much if it's IVs are bad. I xfer lots of mons with high cp but weak IVs. Also, I'll xfer mons with nice IVs/CP if their move set is useless.

However I do make exceptions. I've caught 3 Snorlaxes. All terrible. IVs, cp, moves. Except the third I recently caught had 1700 cp. 33 rating and not very good moves. I kept it but that's a personal choice.

I keep mons with 80%+ rating and good moves.

Keep in mind I'm not necessarily making the best choices. Do more research by searching similar questions.

I had a Dragonite appear at my place once. Caught it. Bad IVs, 3-400 cp and its moves I don't remember but I xfer'd it. I had no candy and I'm sure I'll get a better one eventually.

Most important thing for a good pokemon is the moveset, so just keep evolving the highest CP ones until they have a good moveset, because for example an Exeggutor with Solar Beam and 50% IV is still better than one with Seed Bomb and 100% IV. IV is an end-game thing, higher the IV, higher the pokemon's maximum potential, but still, for example I evolved a 100% IV Magikarp, but the Gyarados that came out of it had Twister! Instant transfer, no regrets. But if it had Hydro Pump? Best Gyarados.. ever

most people say it wants a perfect IV pokemon but he majority of then dont know how it really works... using the gyarados example the the fellow above me, a max lvl perfect IV gyarados will hav 2688 CP and a 1/1/1 iv max lvl gyarados will have about 2330 CP, the difference is much lower on lower lvl, and its just that. CP is not a ramdom number, it's the compilation of the pokemon stats, thats why dragonite have the higher CP in the game, because it has the higher overall stats. So, the first gyarados with twister will be weaker than the second one with hydropump, the first one it's like a very strong guy beating with little slaps, and the second one its a everyday regular normal guy giving a good full hand punch.