Why? Why does the adorableness that is Wooper evolve into this ugly thing? Ugliness aside, Quagsire is quite capable.
It seems to be a hard rule that Water-Ground types are bulky. Swampert, Gastrodon, Quagsire are all shining examples of bulky pokemon. While 95/85/65 defenses may only be above average physically and below average defensively, that is not where the true worth of Quagsire’s bulkiness lies. Nor is it its typing though a lone weakness to Grass vs 1 immunity and 4 resistances is certainly very good. It lies in the ability Unaware. This allows Quagsire to resist all sorts of setup sweepers. Dragon Dance Kingdra? Blocked. Swords Dance Talonflame? All washed up. In the right setting, Unaware Quagsire is a threat.
Booster Denial
Quagsire (Unaware)
Item: Leftovers
-Recover
-Yawn/Haze
-Earthquake
-Waterfall/Toxic/Stockpile
Nature: Careful (+SDEF, -SATK)
EVs: 252 HP/252 DEF/4 SDEF OR 252 HP/196 SDEF/56 DEF
The premise of this set is simple: Tank “boosted” hits and do whatever you want to them. Recover is a necessity, greatly increasing Quagsire’s survivability. Yawn and Haze are two excellent P/hazing options that Quagsire has allowing it to sabotage setup sweepers that it cannot take out on its own. Earthquake is a mandatory STAB move and has an excellent resume to boot. Your last move is up to preference: Waterfall is an alternative STAB move that can hit foes that Earthquake cannot, Toxic is to emphasize the Stalling nature of Quagsire and Stockpile is an effective defensive boosting option, as your opponent cannot match you boost for boost because of Unaware (don’t use with Haze).
Other Options
-Amnesia/Curse
Other boosting attacks that Quagsire can use: one patching its horrid SDEF, the other making it more dangerous on the physical front.
-Swagger
Just evil: your opponent receives double powered confusion, while getting no benefit against Quagsire
-Stone Edge/Body Slam
Other physical options
-Aqua Tail/Ice Punch
Other physical options, but requires transfer
-Acid Spray/Eerie Impulse
Two of the sabotaging options Quagsire has, though cannot be used with Unaware due to the fact that Quagsire will ignore its own efforts.
Countering Mud
Quagsire is far from perfect as a wall. Grass Attacks will maim if not end Quagsire, even more so if they are special. Strong special sweepers can put down Quagsire quickly, especially given how few of them actually rely on boosting moves. Quagsire is slow as hell sitting at a Base 35 SPD. Also if you don't use any setup sweepers, Quagsire loses its raison d'etre. Toxic and Burning compromise Quagsire's bulk, with the later hurting its offense as well.
Finally, you might be Unaware of this but Quagsire receives stiff competition from Clefable as an Unaware wall.