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Looks comes first, Breloom and Probopass for example will never get into my team, ever.
2nd we have typing. Always makes sure to have a Water, Dark, Bug and Psychic in my team, 2 combined prefered. At most 2 of the same type, rest gotta be spread for coverage.
Ability > Movepool. I prefer a prankster Whimsicott over Cacturne with his great movepool.
After that it all comes down to combining, usually 2 and 2, Trick Room Slowking with No Guard Golurk or Tailwind Whimsicott with Guts Heracross.