In order to help
Mewdraco get an answer that is sufficiently different than mine, as well as help other, newer members, I'm copy pastaing my original answer here. :)
Round tends to work best in the triples format, where you can have 2 callers prior and boost it to truly nuclear levels, or to at least inject some unpredictability into who is going to call round. Plus, the boost to round only occurs within a single turn, which requires that both of your pokemon use round (predictably) every turn in order to get that boost.
Mega garde really wants hyper voice for doubles. If you can't use hyper voice (gen 6 gardevoir, for example), I'd recommend giving your mega slot to another pokemon and using a choiced gardevoir instead.
With choice scarf (one of my favorite sets), you can get the jump on and OHKO most dragons (not salamence, though--those tend to run choice scarf too). Problem is, scarfdevoir tends to hit most things with 2HKOs instead of OHKOs, so you have to plan accordingly.
Another great set is choice specs, which hits super hard. Specs garde really needs a lot of bulk in order to work, as it'll generally have to take a hit before OHKOing threats.
Also, psychic over psyshock, because most things you'll want to hit you'll hit harder with psychic. Usage stats: psychic #1 65.4%, psyshock #6 25.8%.
Also, focus blast is kind of a waste in doubles, as the thing you'll most want to hit--mega kanga--will destroy you with sucker punch. Shadow ball, which at least does SE against aegislash and other ghost types, not to mention mirror psychic types, is preferable (to compare, focus blast isn't even on the usage pie chart for garde, while shadow ball is #7, 25.5%).
Personal experience, my favorite KB gardevoir set is:
gardevoir @ choice scarf
Modest 84 HP / 196 Sp.Atk / 228 Spd (
Eggy Emporium's "Dragon Slayer" Spread)
- Moonblast
- Psychic
- Shadow Ball
- Coverage (Psyshock / Focus Blast / Energy Ball / Tbolt)
Either way, gardevoir really likes having quick guard support, as it's biggest fear is sucker punch from mega kangaskhan and mega mawile. Meowstic-m is a good choice, though there are many others.