I will give you pros and cons of each Bug Pokemon.
Leavanny
Pros: Wide movepool, high Attack and Speed for possible sweeping
Cons: Terrible type combination, mediocre defenses, making for probable OHKO's
Scolipede
Pros: Good Defense, High Attack and Speed for possible sweeping, fairly wide movepool (Earthquake= teehee), can set up entry hazards/
Cons: Mediocre Sp. Defense, low HP
Escavalier
Pros: Amazing Defenses and Attack, great type combination with 9 defenses and 1 immunity, can use its slowness for a Trick Room team/
Cons: Extremely slow without Trick Room, it cannot learn Gyro Ball, very weak to Fire, limited movepool
Galvantula
Pros: Great type combination with only two weaknesses, High Sp. Attack and Speed for possible sweeping, ability Compoundeyes increases accuracy of Thunder (depending if you teach it to him), okay movepool/ Cons: Poor Defenses, making for probable OHKO's
Hope this helped!