Definately mage/priest. I can easily accomplish things that the other class combos you listed cannot.
M/P - Excellent DPS(valued in groups), good survivability (Pvp and solo wise), I haven't gotten bored with it yet lol.
P/M - Poor Dps(A healer in most groups), good survuvability (pvp wise), lack of dps prevents most solo play.
S/P - not too familiar with this combo*
P/S - Okay dps, but not great(your group members would probably prefer you on S/P), good survivability, you'll be hated in pvp becasue most people cringe at the sound of an ice blade.
I'm personally a M/P. With that combo, I can solo burn many bosses before they kill me. If the fight takes a while, I have the ability to heal myself. I spent 55min solo'ing 1st boss in Sardo Castle easy mode, and the only reason I was able to do that was becasue I was healing myself. In siege war, you'll have many options available to you: Nukes, roots, dots, insa casts, aoe's. If played correctly, you will be a force to be reckoned with.
That's just my take on this.
http://forum.us.runesofmagic.com/showthread.php?t=75931
The above link shows me, a m/p, in pvp. Check the channal out to see what else a m/p can do