I love to mark players based on what they can do. For example, I mark smashers as green saber, bubble stuns as star, main heals as target, pyros / anni maras fire, slingers as a gun.
This way you just type the target and hope people understand, it's very helpful when a warrior leaps to you to know what it is capable of w/out having to check his stance. That's why I mark the different specs.
Priority:
1. bubble stuns
2. Every other healer
3. Smash dps (depending on what class I'm on I'll stun / snare / peel this class)
4. Pyros
If I'm stealthed I'll go for snipers first, with recklessness and 5 second resil it's almost an auto kill and you save your teammates the trouble of getting to him.