Swalot

Poison-types typically don't get much use in PvE, and Swalot isn't going to change that. Poor offensive stats and an inferior moveset makes this guy relatively useless, being a dex filler at best. While its mono-Poison typing and respectable bulk might make it appealing for gym defense, it should be noted that popular and powerful legendaries like Mewtwo or Groudon will make short work of Swalot. 

Best PvE Offensive Moveset

• Infestation lacks STAB, but synergizes best with Swalot's charge moves.
• Rock Smash has no use. 
• Mud Shot is a good move, but it doesn't synergize with Swalot very well.
• Sludge Bomb is the best charge move for Swalot on offense, and benefits from STAB. 
• Gunk Shot also gets STAB, but has inferior performance. 
• Ice Beam doesn't benefit from STAB, and thus falls behind.  
• Acid Spray is strictly for PvP. 

Best PvE Defensive Moveset

• Infestation and Rock Smash both have their advantages, though the former tends to pull ahead a bit due to its counter-coverage.
• Mud Shot is too weak to be good on defense.
• Sludge Bomb gets STAB and hits neutral attackers hardest, and is also the most difficult to dodge.
• Ice Beam is preferable for defense since it threatens Swalot's weakness to Ground-type attackers.
• Gunk Shot is difficult to access on defense as a 1-bar move. 
• Acid Spray is strictly for PvP. 

Mud Shot doesn't have good damage output, and is primarily used for its high energy generation, letting Swalot access its charge moves sooner. Rock Smash provides wide coverage as a Fighting-type move, but lacks STAB and has poor stats, especially with energy gains. 

Sludge Bomb costs the least amount of energy and benefits from STAB, making it Swalot's primary charge move. Ice Beam offers Ice-type coverage, which can be useful against Swalot's weakness to Ground-type Pokemon. Gunk Shot is typically not recommended due to its expensive energy cost, despite being the most powerful move available if unshielded. Acid Spray is mostly a gimmick to provide value when the opponent would shield a charge move, as it deals pitiful damage when not shielded which may lead to inconsistencies.