I looked up a list of "known" combos but neither one I tried worked so I'm going to start writing down mine. I'm a masochist for minmaxing when I don't need to and want to power up some Lucky pals.
Each pal has a rarity value. The lower the # the more rare. The egg is always the closer # to the average of the 2 pals breeding, after rounding. So pal A being 100 and pal B being 175 (100+175)/2 is 137.5 but rounds to 138. So the egg will be pal B. If the value is equal distant from both it will be whichever comes into the dex first.
This also means you can never breed a pal with a rarer value than the rarest of the 2 breeders.