I use Reliant. Setting up service online takes 5-10 minutes, and service can be activated shortly thereafter if the property has a smart meter. There are no contracts (if you don't want one), rates are competitive, and deposits aren't required if you have good credit or after you have established a track record with them.
ONE CAVEAT - make sure you use the same user ID for all properties. If you use a new ID on each property, there is a higher chance you will have to pay a deposit.