EDITED: long overdue, i've added some interesting sets courtesy of other posters. Hope this helps out.
Quagsire returns, most likely to UU, but with some new toys that could make him even more awesome than he previously was. Why make a topic devoted to quagsire? Partly because he's my favorite pokemon, but mostly because i think unaware being added to his skillset gives him a whole new role in the lower tiers.
Base Stats:
HP 95
Atk 85
Def 85
SpA 65
SpD 65
Spe 35
Abilties:
*Damp: Prevents the use of Selfdestruct or Explosion. Prevents Aftermath from doing damage.
*Water Absorb: Heals 25% HP when hit by a Water-type attack.
*Unaware: If a Pokemon with the ability Unaware is the target of an attack, all of the attacker's [Special] Attack boosts are ignored. If a Pokemon with the ability Unaware is attacking, all of the defender's [Special] Defense boosts are ignored. If an Unaware Pokemon is using Punishment, stat boosts still factor into its power. If an Unaware Pokemon hits itself in its confusion, it still keeps its altered Attack and Defense stats.
New toys
- Unaware. Water Absorb is great, but so is this. Belly Drummers, Dragon Dancers, CM users, and SD set ups become much less threatening when you have something like this in tow. In addition, Quagsire gets excellent support moves in yawn and toxic to punish opponents forced to switch as their stat boosts are meaningless.
- Boiling water. Exactly what Quaggy wanted this generation. After a burn, quagsire becomes quite difficult to kill physically, thanks to his access to recover. Whats more, setting up instead of attacking does you opponent no good if you're running unaware.
- Level ground. You won't see this on my set(s), but it could have some uses because of its speed lowering capability. Though Quagsire will probably still be slower.
it doesn't get many other moves this gen. Though it got recover in HG/SS, so i can't complain much.
Possible movesets
Unaware sweep blocker
Quagsire @ leftovers
Unaware
Relaxed 252 HP 4 Atk 252 Def
-Earthquake
-Boiling water
-Toxic/Yawn
-Recover
This set serves to block potential sweeps, most notably from physical sweepers boosted by DD, SD, bulk up, or bellydrum. Earthquake is your standard damaging move. Boiling water does some damage (albiet, not alot) and has a chance to burn physical attackers, Toxic and Yawn are for your calm mind opponents, who aren't particularly bothered by boiling water or earthquake. Recover keeps quaggy alive longer, and gives him more shots at status-ing the opponents team. Relaxed is used over impish to prevent a drop in power to boiling water. While having earthquake as his only real attacking move may scream setup fodder, his ability makes this set actually work out.
EDIT::
Dragon Killer
Quagsire@ Leftovers
Impish 128 HP, 252 Atk, 128 Def
-Earthquake
-Ice punch
-Toxic/Yawn
- Recover
::
This set (courtesy of Rykard) swaps boiling water for Ice punch, and gains the ability to kill dragon types try to set up on you. with 252 Atk, he can OHKO salamence, and can still survive an outrage from max attack mence. While he won't be able to OHKO pure dragons, or our new dark/dragon hyrda, you should be able to deal some damage while they "set up" as they won't know you aren't running water absorb. As always, a status move is available for situations in which ice punch, and earthquake aren't viable.
A set that has worked really well for me on PO has been
Quagsire @ Leftovers
Careful (+SpDef, -SpAtk) or Impish (+Def, -SpAtk) *I use Careful
Unaware 252 HP/252 Def/ 4 SpDef
-Curse
-Waterfall
-Amnesia/Return/Earthquake
-Rest
Quagsire @ Leftovers
Trait: Unaware
EVs: 216 HP / 108 Def / 184 SDef
Calm Nature (+SDef, -Atk)
- Stockpile
- Recover
- Toxic
- Boiling Water
The EVs and Nature give 385 HP and 233 in both Defenses, which go up to 582 after the third Stockpile, up to the point almost no Grass-type move can OHKO the Water Fish Pokémon (Say, nearly anything that isn't called Leaf Storm, and possibly some of these ones too). If needed, I can BP some Calm Minds from my Espeon. With that, Quagsire becomes almost impervious to special attacks while being able to do reasonable damage with Boiling Water
If it were faster, I would say that Quagsire could have been a surefire counter to Inconsistent pokémon, but they can set up a Substitute before Quagsire can Toxic them, so...
He's an excellent counter to Roobushin (Not sure about SubPunch sets), Doryuzuu and Ulgamoth
I've been using Unaware Quagsire and it is so good it's ridiculous.
Quagsire@Leftovers
Bold
Unaware
128 HP 252 Defense 128 SpD
~Encore
~Recover
~Toxic
~Scald
The biggest point of contention here is weather dream world abilities are compatible with egg moves, as Quagsire gets both Recover and Yawn from egg moves. If not, unaware quagsire may not be as helpful as the water absorb variants. EDIT: issue has been solved, pokemon can have dreamworld abilities and egg moves.