The "ultimate" Rikti mothership raid character isn't a Defender. It's a Controller. Specifically, a Gravity Control/Electrical Affinity Controller. Electrical Affinity is the best support set, bar none, for mothership raids between Energizing Circuit, Faraday Cage, Empowering Circuit, and Amp Up. Gravity Control for Wormhole and Singularity, both of which will help pull Rikti to the mothership's bowl. You can take Fold Space on top of that if you'd like for even more bringing Rikti to the bowl goodness, (but it's not necessary).
That said, if you want to go Defender, Electrical Affinity/Electrical Blast is definitely a good way to go. Water Blast would be another. IMO, Water Blast is the best ranged damage set for mothership raids, between all of the ranged AoE and knockdowns. I like to lead Rikti mothership raids on a Water Blast/Force Field Corruptor. Force Field for all of the defense buffs and very good AoE mez protection. Corruptor because Dispersion Bubble actually gives better AoE mez protection on Corruptor than it does on Defender, and I personally find playing a Defender pretty boring. YMMV. Water Blast because I have to sit in the middle of the bowl for both Dispersion Bubble to be effective and to see what everyone is up to while I lead the raid, and Water Blast does a lot of ranged AoE damage.
IMO, if you're going Electrical Affinity, then I'd go for Incandescence Destiny personally, as Faraday Cage is large enough to cover the entire mothership's bowl. Incandescence will allow you to teleport the league around... especially helpful when people start to stray to the wrong pylons.