There really is no *best* answer, that's just how the game is.

As for some basic tips though, it mostly depends on your monster's stats and your personal playstyle. Plants have a mix of POW and INT techs and tend to be more focused on a defensive playstyle, though you don't *have* to raise it that way.

The exact mechanics of combining unfortunately aren't documented as well for the first game as they are for the second, but basically you want to raise the second monster to have a similar spread of stats. Again, what breed would do best depends on how you raised your plant.