Well let's look at each class . IMO you need a tank heals and cc to be effective as fc groups . Mobility is also a big factor .
For just taking damage the clear two choices are warrior and guardian guardian trades a lil bit of mele mitigation for much more mobility and a larger health pool .
Now ow let's look at heals
priest - burst mitigation with bubble and very high cc with roots or psyfeind and fear . LOWEST MOBILITY and squishyest
shaman -increased health which always helps, shear and purge , over all very low cc . Tanky decent mobility . CLENSE
rdruid - roots , extremely high mobility , second squishiest,
hpally - clense , mobility , tanky , kinda low cc compared to priest but higher than Druid and sham , high heals !
Then the cc
Mage , high cc but breaks easily , high mobility if speced , doesn't dr with any other cc from heals aoe ability
lock medium mobility, cc doesn't break as easily . Dr.s with priest
hunter , high cc , mobility , damage , roar of sacrifice ,
For a mobile fc comp
for cc IMO I would rather have a hunter
heals I'll take a hpally
tank if say guardian
Stationary I'd say
warrior
hpally
priest