I have a Sirfetch'd in Pokemon Sword. His current moveset is Night Slash, Poison Jab, Leaf Blade, and Close Combat. He is currently holding a Leek. So, should I keep Close Combat or substitute it for Meteor Assault?
Close Combat.
Here's why. Close Combat has 120 BP, and Meteor Assault has 150. However, you must recharge after using Meteor Assault, so it essentially has a BP of 75 per turn. While Close Combat lowers your defenses, it doesn't leave you completely vulnerable like Meteor Assault does, and as such it is far better.
Hope I helped!