In R:TW I simply adore Athens which, in my opinion, is indisbutably one of the richest cities on the map. Even after its share of military upkeep charges have been subtracted from it's funds, it still can produce a massive 3,000+ denarii per turn. In all honesty I've never seen one city add quite that much to my coffers.

In BI my love lies with Salamis (that island North of Egypt). At the start it seems a tad pathetic, but, after building and upgrading the port, income from it is epicly vast and the cost of the construction cost is paid back within the turn. Sadly the sea being its only income makes it vulnerable to blockades which devestate its contributions.