The best party composition will always be personal preference and what your expecting to get out of using Demonbuddy.
Personally, I usually end up with a meta group at the end of the season due to the higher greater rifts that can be achieved.
If I am farming for gear for a particular character within that meta group I usually use four of the same class to increase my odds of getting closer to perfect items.
So it all depends on what your goals are.