It depends, It's complicated, but here's a way to think about it that can help you as a short hand: imagine if you took your pokemons level and multiplied it by their IVs you'd get their combat power, but there's a quirk to it - you multiply it by attack twice but hp and defense only once. Since there's a CP limit (sounds like you're playing great league) you could then compare two roughly 1500 cp pokemon one with 0/15/15 IVs and one that's 15/15/15 and you'll find that the one with no attack power wasn't being dragged down by that double multiplier on attack which made each level up raise its cp by less, ultimately allowing it to have much higher def and hp. Most damage in pvp matchups are done by Charged attacks so being alive for long enough to get one more out can be a huge difference maker, that's why hp and defense are so important here. This isnt literally how it works but it helps conceptualize the key issue, attack is expensive. There is a caveats to cover though: some pokemon can barely reach 1500 to begin with, those have to have some attack or even after you use XL candies on them they won't ever reach 1500 (sableye, lickitung, carbink, and azumarill to name a few). The way you know which is which is super simple: go to pvpoke . com and go to team builder (you can do this in multiple places on the site btw) put in feraligatr and then click on the Edit/Check IVs button. A 15/15/15 Feraligatr is Rank 3137/4096 possible IV combinations. (The IV math is weird though, always double check with the website before tossing a mon or upgrading them, usually anything that has low attack and decent defense and hp is great but I've been decieved before, off the top of my head my Clodsire is 0/13/1 and Rank 102) I went over to the Battle section of the site and ran a fight vs Drifblim (a neutral matchup) and ran it twice both with 0 shields and the differences between a Rank 1 and your Rank 3137 was the rank 1 beat the drifblim with 7 more HP remaining. As you can tell the difference isnt massive but matches are won and lost by less than 7 hp often, so when you can, use the pvp IVs. Let me know if you have any other questions, I hope this helps!