If you're interested in running instances and such (PvE) going with a Druid or Paladin + 4 shaman is a very easy way to get into the swing of things.

The learning curve is there, but you can get to the point of being functional very quickly. As you play you will find there are a lot of subtle nuances to how group synergy works together - but the gist of it is that you have a tank and 4 characters that can do ranged DPS as well as heal. The reason for shaman being easiest (IMO) to get your feet wet with is due to the wonderful range of utility that the totems provide, especially when you lay down 4 sets of them.

From your list of group comps and the question about which would be easiest i'd go with:

Priest + 4 warlocks (a bit squishy but tons of fun outside instances)
5 shaman (learning one class + learning control of 5 toons seems like a nice place to start).


Running an all melee team is very doable - but may be an excercise in frustration if you are working on learning all the other mechanics of the game (not that it's super complex or anything, but again there are plenty of kinks to work out when rolling with 5 'toons).

So much of it comes down to personal play style that I think. Leveling doesn't take very long in this game, so if you get one group up to the 50s/60s and decide that they jsut aren't your thing you can just transfer wealth/items and start over with a team that sounds more fun