So here I'm going to list every set I perceive as good
so far to try and give this thread a little more guidance. I'll probably edit this into the OP later.
Note that none of these sets involve the move Transform, boosting your opponent's Attack stat with a frail sweeper that already has Swords Dance, or Curse passing.
tl:dr - most of the sets in this thread are awful. i don't expect every set to be a new standard sort of thing but please at least make an effort
Maci12 - Specs Lanturn
Why it's GCMS material: Not standard (at least in UU) and solves Lanturn's biggest problem (can't do anything back to special attackers) without hampering its role too much. I would have included the Nidoqueen set too but that was just a subtle EV change.
Gone to the beach - SubSeed Roserade
Why it's GCMS material: It's completely unexpected and has a unique niche as a fast Sleep inducing SubSeeder. When you consider how most people expect Spikes or Leaf Storm off the bat it does pretty well.
JolteonFTW - Scarf Meloetta
Why it's GCMS material: A very unique set, so it's creative, but it is also effective and thought was put into it. The set really capitalizes on Meloetta's Base 128 Special stats and decent Speed, making it a useful utility counter and revenge killer.
Ernesto - SubSalac (non-SD) Heracross
Why it's GCMS material: It's a slight deviation from the standard that, while not stellar, has sweeping potential in specific situations that ordinary Heracross does not. Dropping Swords Dance is really a big help for coverage. The Roserade set is pretty good too.
WhiteQueen - Trick Room Victini
Why it's GCMS material: Turns Victini into a late game sweeper, how cool is that? Seriously though, you can just get rid of your opponent's Fire resistances and go to town with Victini. A completely different role than most battlers expect and it can be really useful on the right team.
sorry warb didn't include houndoom since that is literally the standard set
Ace Emerald - Mixed Porygon-Z
Why it's GCMS material: More useful in the Chansey days but it still abuses Download to get the leg up on most walls. Gives P-Z a bit more of a chance to shine than it has now. Doesn't really fix its ease of being revenge killed, but not much can.
please note that none of these sets are from the third page of this thread