Soul Level 120/125 is standard for Arena and Fight Club play, but you still get some good co-op play at 140/150. For invasions in early areas, try 40. For mid and very late game, 60 or sometimes 80 is fine. FromSoft eventually adapted the summon range for Blades of the Darkmoon and Blue Sentinels to be broader than at release, so 80 is good and 120/125 also works (to the degree that those auto-summon covenants function at all with the game less populated).
[I haven't experimented with Arena matches at 80 but given that the Ashes of Ariandel DLC is scaled in enemy damage and health for later game character builds, Soul Level 80 could very well be viable, given that this DLC is where you acquire the Champion's Bones to burn and access the Arena.]