I find EV and KP SM the easiest; they drop columi gear and can be a good way to get used to the larger groups.
EV/KP HM are the second easiest. I find that they can be easily pugged, but still might pose some problems to an untrained group. They drop Rakata.
Either EC or TFB would be the next step up. Although they drop the same gear as EV and KP HM, they are more mechanics heavy and would require better coordination.
These would probably be the best ones to start with. EC HM drops campaign, and TFB HM drop Dreadguard, but I wouldn't recommend these until you have better gear, and more experience.