There is a Smogon custom format called Cross Evolution that cares about such things, and to that end they've compiled a spreadsheet of evolution data.
At the high end, Gyarados and Milotic gain 340 points each, but are on the ignore list for some reason. Golisopod is next with a +300 (and if not for losing points in speed, would tie the other two). Noivern and Frosmoth both gain 290, while Solgaleo and Lunala gain 280 in evolving from Cosmoem.
On the opposite end, you have Pokemon that decidedly don't see use in Cross Evolution (though their prevos often do serve as a basis to tack evolutions onto). If you count Nincada->Shedinja as an evolution, that actually loses 30 points overall, thanks to the HP gimmick. If not, of course there's Scizor gaining 0 as the smallest change, and Metapod/Kakuna/Silcoon/Cascoon each gaining 10. Once you get past that ignore list, the other low totals are: Spewpa +13 (that evolves at level 9, not 7), Porygon-Z +20, Silvally +36 (all of which is due to speed), Magmortar +45, and the trio of Vibrava, Electivire, and Rhyperior all gain 50.
