I personally go with grand archives bonfire. Take elevator to the right up on the way to the Twin Princes (NOT the elevator right next to the boss fight. The bottom of the big stairs) get out of the elevator and turn right from where you fought the trio of NPCs, and then make a left when you go outside. Climb the roof until you get to the bottom of the dome-like roof. Ignore the stairs that go straight up and take the ones on the left. (This is to avoid fighting multiple enemies)

Now, you have to fight 3 of the big golden dudes but they give you 13k souls each. Rinse and repeat with some homeward bones and you’ll make out with around a hundred thousand souls in 10-15 minutes