Jolteon 100% TS Thunderbolt (Primary attacker against Vaporean, Gyrados, and Lapras)
I've also invested a decent amount of stardust into a few others:
Slowbro 96% Confusion Ice Beam (Gym defender which I stopped short of maxing once it was nerfed)
Arcanine 96% Fire fang Fire blast (Used to be one of my go to attackers before the nerf and I have stopped powering it up since)
-Gyrados 96% Bite Hydro Pump (One of my most used attackers the only reason I haven't maxed it out is that I think 1999cp looks cool and it's getting the job done anyway)
-Ninetails 98% ember fire blast (Powered up a few times just because the animation looks so cool)
-Rhydon 89% Mudslap Stone edge ( One of my go to attackers powered it up to above 2k but didn't feel the need to max)