Psychic. It is not my favourite type, but I battle best with Gardevoir and Xatu more so than with any other Pokemon.
Assuming that I was an eighth Gym Leader in a region I would use:
Level 41 Kirlia (Trace): Reflect, Psychic, Focus Blast, Gravity
Level 42 Girafarig (Inner Focus): Agility, Baton Pass, Stomp, Zen Headbutt
Level 43 Slowbro (Own Tempo): Curse, Aqua Tail, Zen Headbutt, Earthquake
Level 44 Xatu (Early Bird): Rest, Psychic, Drill Peck, Confuse Ray
Level 46 Gardevoir (Synchronize, @ Lum Berry): Calm Mind, Psychic, Thunderbolt, Hypnosis
I would give out the Insight Badge and the TM for Psychic.
My rematch team would be:
Level 52 Lunatone (Levitate): Reflect, AncientPower, Psychic, Ice Beam
Level 52 Girafarig (Inner Focus): Agility, Baton Pass, Headbutt, Zen Headbutt
Level 54 Slowbro (Own Tempo): Curse, Aqua Tail, Zen Headbutt, Earthquake
Level 56 Xatu (Early Bird): Confuse Ray, Psychic, Drill Peck, Heat Wave
Level 58 Gallade (Steadfast, @ King's Rock): Swords Dance, Zen Headbutt, Close Combat, Rock Slide
Level 60 Gardevoir (Synchronize, @ Lum Berry): Calm Mind, Psychic, Thunderbolt, Signal Beam