I'll try put in a list for easy to hard factions.
Last Alliance - Easiest (obvious reasons)
Ragnars Kin - Easy (beetles are plentiful and provide quite the xp along with two great jobs)
Shadow Legion - Easy/Medium (quite the player interaction along with relic bonus)
Daeron - Medium (jobs and outposts all over the place)
Ancient Ones - Medium/Hard (one "job" and seperated, however two guilds along with slave allies)
The Circle - Hard (relatively seperated but from players and civilization and everything so don't know what to say)
Mystral Ascendancy - Hardest (so seperated and confined in many ways, this faction is the ultimate test for any player's sanity

).
Just my vague ideas, I'm sure some factions are interchangeable on the list.