The best nature for Gardevoir is either Modest or Timid. Both of them lower Gardevoir's useless Attack stat and boost either Sp. Attack or Speed, respectively. The best option between those two depends on the set you plan on running. Choice Scarf and Mega Gardevoir will prefer to run Modest. A Choice Specs set can can run either — it depends on if there are any base 80s you want to outspeed. Mega Gardevoir can go Timid as well, if you wish to edge out other base 100 Pokémon.
TL;DR either Modest or Timid, with Modest being the most useful.