Barbaracle

Barbaracle as a Rock/Water Pokemon has lots of competition from existing Pokemon already in the game, but unfortunately with only 194 ATK and a suboptimal moveset it falls behind current meta attackers. Its stats would allow it to be a decent defender, if not for its double weakness to- and lack of threat against Grass-type Pokemon. Sharing a typing weakness to Fighting-type attackers is also generally not favorable, as many currently meta gym defenders tend to share this problem. 

Best PvE Offensive Moveset

• Water Gun benefits from STAB and is the best overall fast move.
• Fury Cutter provides faster access to charge moves, making it okay if lacking TMs.
• Mud Slap has good synergy, but lacks STAB and falls behind. 
• Razor Shell benefits from STAB and suits Barbaracle's role the most. 
• Stone Edge also has STAB and can work on targets weak to Water and Rock.
• Grass Knot is the strongest when attacking a neutral opponent, but consider other attackers instead. 
• Skull Bash lacks STAB and can't be Super Effective vs anything. 
• Cross Chop is a poor move for offense. 

Best PvE Defensive Moveset

• Mud Slap is the best for defense due to its high base damage. 
• Water Gun and Fury Cutter are very poor for defense, but at least the former gets STAB. 
• Razor Shell hits fairly hard and frequently.
• Cross Chop can be used frequently as a 2-bar move, and is hard to dodge, but doesn't hit very hard. 
• Grass Knot deals significantly more damage as a 2-bar move, but is easier to dodge. 
• Stone Edge might not be used before Barbaracle faints, but hits hard and fast. 
• Skull Bash is too slow for defense. 

Water Gun is an overall well-rounded move that provides a balance of damage output and energy gains. It hits harder with STAB and provides good Water-type coverage. Fury Cutter lacks STAB (and damage output in general), but makes up for this by providing energy quickly, letting Barbaracle access its charge moves sooner. 

Cross Chop costs the least amount of energy, can be used frequently, and its Fighting-type coverage is quite valuable in the meta. Either Grass Knot or Stone Edge can be chosen depending on whether Grass-type or Rock-type coverage is needed, though it should be noted that only the latter benefits from STAB.