Here's a comprehensive list of everything in the game that increases how much you gain in stats after a fight. These items affect only monster encounters, and not other encounters. Because there are only a few items that do this, you may find it more effective to increase Monster Level too. Any fractional stat points you gain will vary randomly between the next integer and the last integer, depending on how close it is to either.
The items in this table list mechanics which increase stat gains by a set amount. In some cases, variable increases may be more beneficial.
This table lists mechanics which impart variable bonus substats after victorious combats.