Amoonguss is one of my favorites. It takes on rain teams so well that you could say "I have an Amoonguss, so I counter rain." Politoed can't Hydro Pump, Toxic or Focus Blast it, Ice Beam does nothing, especially after Giga Drain return factored in, and that's just one move...
Spore can be saved for something troublesome like Hurricane Dragonite, and the other two slots can be customized to suit your needs; Gus has quite a few options. It can run two of Clear Smog, Synthesis, Stun Spore and Hidden Power Fire / Ice to counter stuff. Clear Smog cockblocks stuff like Toxicroak, Synthesis outside weather, alongside Leftovers Giga Drain and Regenerator, gives Amoonguss that unlimited health/unkillable feel.
HP Ice is my favorite coverage move. It has decent synergy with Giga Drain, and other obvious stuff. The poison typing is the best thing about Gus. He's immune to all the Toxics flying around right now, absorbs Toxic Spikes, Resists Fighting and has a Bug neutrality, which is great for taking on Scizor.
My set is Giga Drain / HP Ice / Synthesis / Spore. When Regen is released, Synthesis won't be needed, so you can run a support move, but the other three moves should be standard IMO.