I have a Steelix, not maxed out, but when used in PvP, it's not very good. I think Steelix was intended to be a tank on gyms. Electivire on the other hand I believe has the highest DPS out of all non mythical electric types. I suggest powering up Electivire, seeing that the 7km eggs now include Elekid, PLUS the event giving you 2x Candy and Stardust for hatching. Electivire would probably be the easiest to evolve.
Milotic is one of the best gym defenders in the game, I'm not too sure how they fare in attacking situations or in PvP. I don't know when Mamoswine is being added to the game, but it might be smart to prep for its arrival. I have one strong Piloswine and I'm sitting on just over 100 candies so I can evolve immediately once Mamoswine is added. (It'll be the best Ice Pokemon in the game by far)
Currently, I suggest Electivire, then work on your Swinubs.