If you have ever wondered "Which of these two Dialga/Swampert/whatever should I power up?", here is how you quickly find the answer using PVPoke Battle Matrix (or, how I learned to stop worrying and love the imperfect Abomasnow).

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Go to PVPoke and select battle, then your league, then Matrix Battle then add a pokemon

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Here the hundo has been entered first, now I'm adding my Abomasnow and editing the IVs to mine

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On the right, go to Quick Fill and select the meta you want to run against and then Battle

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You'll get a run down of their respective performance, in this case there were slight differences in performance that I wasn't sure I wanted to live with so I ran it a second time setting both to level 41 (best buddy) - Voilà! My Abomasnow is worth powering up.

There is a little more to it than this as Klibbistorm mentions in a comment below. I hadn't posted initially as I was looking for a fast and accessible post, however I think worth being aware of. The above type of analysis is most suitable for mons where you are comparing to a hundo - either because they max out beneath the CP cap of a particular league, or for Master League/ML Premier - or for where you are manually entering the specific IVs for both. If you run this and do not alter the IVs at all PVPoke will default to entering the IVs for the rank 500 version of a particular mon (you can change it to default to rank 1 IVs in settings on PVPoke).