Good ol' Quagsire, getting overshadowed by Swampert. I will admit many things on the superiority of Swampert vs Quagsire Debate. Swampert does have much better base stats, the option of using both special and physical moves, and Stealth Rock. On the other hand, the Quagsire lead functions as an anti-lead AND comes back much lack Swampert to annoy the enemy.
Quagsire @ Leftovers
Water Absorb
Adamant
252 HP / 252 Atk / 4 Def
-Aqua Tail/Ice Punch
-Earthquake
-Encore
-Yawn
Now. Quagsire has two things swampert lacks, which is Encore and Water Absorb. He matches up excellently against the most common leads such as Azelf and Aerodactyl and slaughters them. Most fast Stealth Rockers will Taunt Quagsire to prevent a Yawn or Encore. Use that chance to pummel the enemy with an Aqua Tail to the face or Earthquake, depending on need. The tricky part is to use an attack or to try to Encore the Stealth Rock. For common leads lacking taunt (Metagross for example), you would Encore the Stealth Rock (even if you get an attacking move instead, you gain a free switch in). Proceed to yawn the switch, thereby revealing probably 3 members of their team at best. This set will also come back later to haunt the enemy, especially any Gyarados lacking Taunt. Water Absorb is free healing against many water users (any Starmie without Psychic, Trick, or Ice Beam automatically loses unless it's a hidden power Starmie). It also can somewhat counter Machamp with Encore or Yawn, as it will never be OHKO'd and can force a switch. The EV's are spread for maximum power and survivability. And you can run a Damp Quagsire for Explosion stopping, but I find Water Absorb much more effective. This set needs a team that can support it well and can provide Rapid Spin and or Stealth Rock.
Against Common leads:
Azelf: If it's trick, switch out to prevent a crippling Choice Scarf. If not, then proceed to pummel it to death with either Aqua Tail. At worst, it'll explode. But they usually taunt and then SR.
Machamp: Cross your fingers and do an Encore if you have a Ghost type to absorb Dynamice Punch, or simply Yawn and switch to a Set-up.
Breloom: Absorb the sleep unless you have something better.
Aerodactyl: Absolutely destroyed by Aqua Tail since it has to taunt first. It can't touch Quagsire in any way.
Metagross: Aside from Exploders and or ones with HP Grass, Quagsire will demolish it with Earthquake and Encore. Encore the SR, either switch or kill.
Swampert: It's brethren usually falls to Encore, since I doubt Swampert can ever OHKO it unless it runs HP Grass.
Infernape: Lead Infernape usually lack Grass Knot and as a result, are easily destroyed. At worst, you'll be Close Combated. Even then, you'll almost never be OHKO'd. Either go for the kill with E-quake once it sets up SR or Encore it.
Jirachi: If you get tricked, you can still pummel Jirachi to death with Earthquake and or use Taunt for a comedic effect, forcing a double blind switch. None of it's common moves should OHKO Quagsire, especially lead Jirachi.
Heatran: Earthquake for the win! Unless it has HP Grass. But either way, an Encore or Yawn with force a switch. I'd Encore the SR, as it thinks it can outrun you. Earthquake will cripple it forever, and anything it does will not OHKO Quagsire except for HP Grass and Explosion.
Roserade: Switch out. Now.
Ninjask: An attempt at Yawning or Encoring will destroy Ninjask if it succeeds. If not, you'll lose a valuable turn. If it substitutes, shatter the subs with Aqua Tail, since if it SD's, then the recipient usually can't hurt Quags that much either.
It also walls Empoleons without GK, Gyarados without Taunt, Electivires without HP Grass, Gliscor, Heatran without HP Grass, Jolteon without HP Grass, Magnezones without HP Grass or Magnet Rise, partially blocks Mamoswine and Weavile, most Scizor, Rotom-W, Starmie without Psychic or Ice Beam, Suicunes and Vaporeon without Ice Beam or HP Grass, most Tentacruel, and Zapdos without HP Grass.

Quagsire @ Leftovers
Water Absorb
Adamant
252 HP / 252 Atk / 4 Def
-Aqua Tail/Ice Punch
-Earthquake
-Encore
-Yawn
Now. Quagsire has two things swampert lacks, which is Encore and Water Absorb. He matches up excellently against the most common leads such as Azelf and Aerodactyl and slaughters them. Most fast Stealth Rockers will Taunt Quagsire to prevent a Yawn or Encore. Use that chance to pummel the enemy with an Aqua Tail to the face or Earthquake, depending on need. The tricky part is to use an attack or to try to Encore the Stealth Rock. For common leads lacking taunt (Metagross for example), you would Encore the Stealth Rock (even if you get an attacking move instead, you gain a free switch in). Proceed to yawn the switch, thereby revealing probably 3 members of their team at best. This set will also come back later to haunt the enemy, especially any Gyarados lacking Taunt. Water Absorb is free healing against many water users (any Starmie without Psychic, Trick, or Ice Beam automatically loses unless it's a hidden power Starmie). It also can somewhat counter Machamp with Encore or Yawn, as it will never be OHKO'd and can force a switch. The EV's are spread for maximum power and survivability. And you can run a Damp Quagsire for Explosion stopping, but I find Water Absorb much more effective. This set needs a team that can support it well and can provide Rapid Spin and or Stealth Rock.
Against Common leads:
Azelf: If it's trick, switch out to prevent a crippling Choice Scarf. If not, then proceed to pummel it to death with either Aqua Tail. At worst, it'll explode. But they usually taunt and then SR.
Machamp: Cross your fingers and do an Encore if you have a Ghost type to absorb Dynamice Punch, or simply Yawn and switch to a Set-up.
Breloom: Absorb the sleep unless you have something better.
Aerodactyl: Absolutely destroyed by Aqua Tail since it has to taunt first. It can't touch Quagsire in any way.
Metagross: Aside from Exploders and or ones with HP Grass, Quagsire will demolish it with Earthquake and Encore. Encore the SR, either switch or kill.
Swampert: It's brethren usually falls to Encore, since I doubt Swampert can ever OHKO it unless it runs HP Grass.
Infernape: Lead Infernape usually lack Grass Knot and as a result, are easily destroyed. At worst, you'll be Close Combated. Even then, you'll almost never be OHKO'd. Either go for the kill with E-quake once it sets up SR or Encore it.
Jirachi: If you get tricked, you can still pummel Jirachi to death with Earthquake and or use Taunt for a comedic effect, forcing a double blind switch. None of it's common moves should OHKO Quagsire, especially lead Jirachi.
Heatran: Earthquake for the win! Unless it has HP Grass. But either way, an Encore or Yawn with force a switch. I'd Encore the SR, as it thinks it can outrun you. Earthquake will cripple it forever, and anything it does will not OHKO Quagsire except for HP Grass and Explosion.
Roserade: Switch out. Now.
Ninjask: An attempt at Yawning or Encoring will destroy Ninjask if it succeeds. If not, you'll lose a valuable turn. If it substitutes, shatter the subs with Aqua Tail, since if it SD's, then the recipient usually can't hurt Quags that much either.
It also walls Empoleons without GK, Gyarados without Taunt, Electivires without HP Grass, Gliscor, Heatran without HP Grass, Jolteon without HP Grass, Magnezones without HP Grass or Magnet Rise, partially blocks Mamoswine and Weavile, most Scizor, Rotom-W, Starmie without Psychic or Ice Beam, Suicunes and Vaporeon without Ice Beam or HP Grass, most Tentacruel, and Zapdos without HP Grass.