
Surprised there isn't a thread on this mon yet, since he's one of the staple defensive pokemon of the tier (and the main reason why everything and their mother runs HP Grass). He has respectable bulk, great abilities (Unaware is the best by far), Recover, good support moves, and decent offenses. His only problem is that he is easily countered by every bulky Grass-type of the tier and is still hit very hard by Grass-type coverage moves, being x4 weak to grass, compounded by the fact that most special attackers in NU have room for a Hidden Power to slam his one weakness. All of his abilities are useful (Yes, even Damp, though it's much less useful than the other two abilities).
Common sets for this adorable little guy include:
Bulky Wall
Quagsire @ Leftovers
252 HP/252 Def/4 SpDef
Relaxed / Unaware
Scald
Earthquake
Toxic/Yawn
Recover
Two good STAB options, a status move, and recovery to keep him fresh. Walls most physical setup pokemon and spreads status until the end of time. This set can also be run with mixed EVs to better handle special threats, but he really likes maxed defense to better handle Absol and Swellow. Both abilities are good, so pick whichever your team needs more!
Curse
Quagsire @ Leftovers
252 HP/252 SpDef/4 Def
Careful / Unaware
Curse
Waterfall
Earthquake
Recover
Standard Curse set, but Unaware lets him come in and set up on a lot more things. His moves get good coverage but still leave him hard walled by Grass-types, so you have to remove or weaken those before attempting to set up with quaggy. Ice Punch is illegal with Unaware, but if you really want to use that you should probably use another curser with different coverage. Even with maxed special defense, two non-boosted grass attacks will generally finish him off, so keep an eye out for special attackers that like to run Grass coverage like Magmortar and Gorebyss.
Gimmick sets I'm suggesting for fun and discussion are:
Bulky Offense
Quagsire @ Life Orb
252 SpAtk/252 Spe/4 HP
Modest / Water Absorb
Surf/Waterfall
Ice Beam/Ice Punch
Earth Power/Earthquake
Recover
Base 65 spatk/base 85 atk can't hurt anything? Tell it to Tangela, Swellow, Absol, Sawk, and the numerous other not-so-specially-bulky pokemon in the tier. He needs max speed to get the jump on pokemon that would otherwise outspeed him, but the higher power of special moves and lower special defense of NU generally makes up for his subpar special attack. Earth Power 1hko's every steel in the tier, Ice Beam 1hko's Torterra, ko's Tangela that switches in on any other attack and SR (and you outspeed standard defensive variants), and it does a number on Eggy and Serp too. It can even 2hko opposing Quagsire with Surf. Unaware is illegal with Earth Power (and Ice Punch if you do a physical equivalent of this set), so Water Absorb is the only real choice, but it helps give it switch-in opportunities and recover health when you don't have the free turn to actually use Recover.
Gambler's Curse
252 HP/252 SpDef/4 Def
Careful / Unaware
Curse
Amnesia
Recover
Waterfall/Dive
Do you feel lucky? One crit can bring this set to its knees, but with the unparalleled ability to ignore opponents' stat boosts, it can become an amazingly bulky and strong mono-attacker if you play your cards right. Maxed Special Defense and Amnesia lets it set up on things that would normally beat it with ease, like Gorebyss' and Magmortar's Hidden Power Grass and LO Kadabra's Grass Knot (assuming you Amnesia as they switch in or as Gorebyss smashes, expecting to beat you easily). It's still inadvisable to set up on actual grass-types though, since Quaggy doesn't get any good moves to hit them with. Waterfall is the best single attack most of the time as it can still hit the birds and other flying threats, but Dive is an option to consider to not only give you an extra turn of Leftovers recovery after setting up, but to also stall for Toxic damage on threats that wall Quaggy even after it's fully set up. Bulky pokemon with Toxic ruin this set, as they can generally stall it out before it's fully set up. Altaria gets a mention as a non-grass counter that can easily stall it out and set up on it with Dragon Dance or Cotton Guard. Pairing this with a cleric pokemon like Miltank, Audino, or Lickilicky is a necessity to heal it from status, and Toxic Spikes are also a good idea to stall out certain other difficult threats like Alomomola.
Let's talk about this derpy pokemon. :)