More GSC mons and sets that have yet to be posted. Because why not.
Muk @ Leftovers
-Sludge Bomb
-Fire Blast
-Curse
-Explosion
Can't believe this hasn't been posted yet, but many people tend to forget that Muk is a thing in GSC. It's very dangerous, though. You'd be surprised at how many teams go without a Rock/Ground/Ghost-type, and even with those in the way, Muk has a chance to get past Ttar/Rhydon/Ground/Misdreavus in the long run with a SB poison (not easy, and takes a while, but it can happen!), and mons like non-EQ Ttar and non-Ptrap Gengar might wall Muk, but can't do much in return. The most annoying mon for Muk to deal with is actually Nidoking, but it can Explode in that case. Muk has other iffy matchups against Raikou/Zapdos and Psychic users such as Starmie that can break it on the Special side after it's Cursed up, but again, that's what Explosion is for, to force trades in otherwise-awkward matchups against crucial mons (including against a Curselax that's setting up alongside you). Muk pairs well with Ttar to help get Ghosts out of the way.
Golem @ Leftovers
-Earthquake
-Rock Slide
-Fire Blast/Curse
-Explosion
Golem is getting pretty popular on PO's GSC ladder and among some newer players, and it ain't bad. EQ is a nice STAB, RS keeps Zapdos from switching/staying in easily, and Fire Blast keeps Skarm at bay. I personally like Curse, though, so I don't have to boom immediately to beat Lax. The result is a mediocre mixed attacker that's just good enough to get a good Explosion trade against Snorlax or Zapdos (or even a Bulky Water such as Suicune on the switch). It can struggle to find good opportunities to switch in due to its typing, and is weaker than Rhydon as a straight-up attacker, but when it does get in, it can do damage. I've also seen some people run Rapid Spin on Golem, typically over Rock Slide since you're typically going to have to Explode on Zapdos anyway, but given how hard it is to get Golem in and how he just baits Cloy right back in anyway, I don't think it's really worthwhile to try to shoehorn Golem into an Exploding role.
Smeargle @ Leftovers/MiracleBerry
-Spikes
-Spore
-Mirror Coat
-Destiny Bond
Pretty sure this has been posted in a Smeargle-specific thread located deep in the archives of RoA, but I'll post it anyway. This is pretty unconventional for Smeargle, especially considering that it's usually a super-dangerous Baton Passer. I don't know the item, I'm not used to picking items other than Leftovers but Smeargle is one of the things that lasts for such a short amount of time that MiracleBerry might legitimately be a better item, even if it's not going for the one-time AgiPass kill shot. It sets up Spikes really well in the lead position, at least in the short term, thanks to how unexpected this set is and how it has access to Spore to put a spinner to Sleep. Mirror Coat is also a fun surprise to spring on a lead Zapdos (really common), as bouncing back a Thunder that lands on Smeargle will OHKO Zapdos for sure. Leftovers give a chance to recover back lost health from the Thunder, whereas MiracleBerry stops Para from preventing Smeargle from Destiny Bonding for the 2-for-1.
Smeargle @ MiracleBerry
-Agility
-Baton Pass
-Spore
-Substitute/Destiny Bond/Belly Drum/Spider Web
Another Smeargle set, this one being the Standard! It's not a standard mon, though, so I figure it fits here. This guy is reserved for BP-specific teams, and aims to go for the insta-win, JoltWak style. With Spore, it stops any single Phazer that might get in its way in its mission to pass Agility to an otherwise slow-but-dangerous sweeper. MiracleBerry allows it to lead against stuff like Nidoking without being crippled by Sleep, or Zapdos without being crippled by Paralysis. The last move is a toss-up. Substitute can block Sleep and status that would otherwise be aimed at your BP target if you were to pass on that turn. Destiny Bond allows Smeargle to potentially do something useful after it takes a hit while passing. Belly Drum can make whatever receives the BP even more dangerous if you can pull it off. Spider Web is super-dangerous and, afaik, legal in Smogon's GSC format (only Sleep Perish Trap is banned, nothing about Sleep Trap Pass). If they lead something slower than Smeargle it's basically gg.
Heracross @ MiracleBerry
-Megahorn
-Earthquake
-Reversal
-Endure
What's with all these non-Leftovers sets? EndRev Heracross seems like a noob set, but it's actually legitimately dangerous, even after people are totally expecting it after they see your lack of Lefties recovery. Max power STAB Reversal can outright sweep teams that lack a resistance (and, in fact, many teams just take straight-out Super-Effective damage from the move). Bae 85 Speed is also pretty darn good for GSC, and Megahorn stops those Psychics (Starmie and Exeggutor) from walling your Reversal. Earthquake as the coverage move pounds on stuff like Gengar and Nidoking that can otherwise make sweeping difficult. Rapid Spin support is a must with this thing, whereas Pursuit and Paralysis support are very nice for making a sweep much easier. Also helpful is having a really good way to take out Zapdos, since that it EndRev Heracross' main enemy.
Zapdos @ Leftovers
-Thunder / Thunderbolt
-Hidden Power [Water] / Hidden Power [Ice]
-Thunder Wave
-Rest
I don't know how "underrated" this set really is, but considering how RestTalk is far and away the "main" set, this seems like it could belong. I think Thunder and HP Water are best for getting high damage on threats that might be immune to/not mind paralysis (Steelix and Snorlax, mostly), although Tbolt is always nice for its accuracy and HP Ice for actually hurting Exeggutor instead of just paralyzing it. Zapdos can be really difficult to deal with, and because of that it can paralyze half an opposig team during its reign of terror. Rest can be iffy on a Zapdos without Sleep Talk, but it prevents Zapdos from being totally screwed by Paralysis or Toxic.
Houndoom @ Leftovers
-Fire Blast
-SolarBeam
-Sunny Day
-Pursuit
I know the set posted on-site has Crunch and Counter on it, and while Counter can be loads of unexpected fun, Crunch doesn't do a whole lot for Houndoom except make Pursuiting Ghosts slightly easier (Fire Blast still does a ton, though). I like this set, personally, because it might allow Houndoom to let loose a Sunny Day sweep against a team with a weakened/sleeping Snorlax, all while allowing it to put Exeggutor on a very short timer thanks to Pursuit. Struggles with Tyranitar because Ttar can OHKO with Rock Slide and Houndoom can't hit it with Solarbeam on the switch. For this reason you could try HP Fighting over Pursuit for a pure sweeping Houndoom.
Moltres @ Leftovers
-Fire Blast
-Sunny Day
-Rest
-Sleep Talk
Hey, another Sunny Day-abusing Fire! There's not much you could try with Moltres' other moveslots, especially considering that a coverage move like HP Grass isn't all that effective without a Growth Pass and does almost as much to Waters as a Sun-boosted Fire Blast. Thus, RestTalk is probably the best use of the last two slots by making Moltres more status-proof and allowing it to make use of its Zapdos-like bulk. Moltres can tank Lax, unlike Houndoom, and can easily 3HKO Lax in return with Sun-boosted Fire Blast. It struggles more than Houndoom does against Waters, though. I suggest using Moltres with Growth passers such as Espeon, Jolteon, and Vaporeon to make the most of its Fire Blast (in particular, with a Growth boost, its Sun-boosted Fire Blast can 3HKO Vaporeon and Suicune). Ttar, again, is an issue, although again, RestTalk could be eliminated to allow room for HP Fighting (or even HP Grass) so Moltres can hit it on the switch.