for our veterans, this is common sense i guess, but it might be an interesting read for our new players or those who just enjoy reading allods-related things
usually, due to nature of certain classes, some get targeted more often than others
(exceptions always exist, like low geared but this is not the topic here, we assume ~equal gear & runes and as well traditional specialisations, so no "tank psi" etc

)
now, how does it go?
i assume our fictional battle takes place in some battleground where many combattants are relatively close together with an open way to retreat; arena of death or battle for cargallas (BFC) in siegemode for example, but also dominion-maps
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Tanks
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worst target imagineable is a 1H/shield paladin with fervor, divine foresight seal and a healer backing it up or a similar warrior; these are only worth attacking when no healer is near or if they already have woundcomplexity
since they are in defensive spec their dps is usally lower than the dps of non-tanks, so its not a big problem having them around
so why do not attack them? because in the time we attack them the dps'ers of the enemy team have time to attack us without beeing disturbed
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Healers
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another hard target are the fullhealers, theoretically healers should be killed first but as soon as there are more than 1 it gets hard and maybe is only a waste of dps; it would be better to distract / cc the healer and dps on the "squishies"
their bubble/ghostcover also interrupt the dpsflow in a serious way
as annoying their heals are, they usually deal close to none damage and thus are not a direct threath
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Damagedealers / Controllers
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some of these are the fastest (best) targets, especially those with no "second life"
scouts and sorcerers can be easy killed with burstdamage, even a backuphealer cant stop this happening for long
they have no proper second life
(scouts last laugh is kinda second life but only lasts for a few seconds and then he dies anyway while mage can only flee with 3xteleport+invis after stonewall/elem. shield is gone)
bards&engineers&wardens have no real second life but before they die they get some more HP and / or a massive damageabsorption which will keep them alive some more time to retreat or flee or kill the attacker or w/e
psi has a special skill, the timeloop, which reverts the HP / mana / effects from 10 seconds before, however this needs to be done manually and cannot be used under CC
even these classes can be built as remarkably resistant but then lack a lot of dps
so what would be a general, good priority for attacking classes?
there is no universal answer to that due to diversity among the playstyle and lots of other circumstances, but i still dare to make a list (suggestion!)
1 Sorcerer (dies fastest due to cloth-armor once barriers/illusions are gone, if focussed no chance)
2 Scout (similar to sorcerer, just a bit more defense due to leather-armor and more defensive skills)
3 Warden (pet can disrupt casters and warden has proper heals, so kill him fast!)
4 Psi (massive CC is very interruptive)
5 Bard (best AOE class & supports enemy team, but only really matters if standing close)
6 Engineer (has good cc but in masspvp matters not much)
7 Necromancer (has better heal over time (HOT) and can easy keep a lot of players in permaheal (dark empathy&plague of mending)
8 Healer (heal over time cant be kept active on multiple targets due to manacosts for multi-HOT)
9 Warrior (dies faster than paladin, cant absorb damage fully, 50% of dmg go through directly)
10 Paladin (most resilient target, can absorb dmg better than every other class)
(for those who wonder why i dont put healers in 1st priority: in the time the healers get attacked, bubbe away etc so much time passes, the enemy DPSers can do a lot of harm while the healer cant keep alive the non-tanks for long if proper focussing is done!)
usual deviations:
healers in dpsspec (no longer fullhealer, becomes -> dps)
warriors with 2handed weapon (no longer tank, becomes -> dps)
paladin with 2handed weapon (no longer tank, becomes -> dps)
rare deviations:
scout in tankspec (reduced damage & increased survivability)