There are too many mobs in the game to have an all-inclusive list, and people rarely write guides for trash in raids.
It's a good question though, and one not terribly appropriate to this thread.
Maybe I'll write something up in a thread of its own once I've given it some thought, but generally you'll want to learn each instance and get used to identifying who are the biggest risks/priorities for yourself, and once you've been a tank you can start to take notes from people who tank for you on what their priorities.
Usually it'll be something like this:
1.) Does it heal? Probably one of the first you'll want to kill unless you're totally confident you can keep it locked down indefinitely *and* you actually want to.
2.) Can it be tanked, can it be tanked indefinitely? In AN for example, Skirmishers will at a certain time length, pick a random target and lock on, becoming untauntable. You'll want to nuke them before they do so. This isn't common, but it can be an issue.
3.) Does it represent a particular damage risk to the tank or the group. Many casters can meet this. While not heals, the spike damage from their spells can be threatening on the tank, or maybe they AoE the group. Taking them down early on will save healer trouble. (Some casters are composites of 2 and 3, where they'll just cast at random targets in the group, usually for less damage, but you don't want them hanging around).
4.) What will die fastest? Sometimes the big bruiser may be more threatening, but the little guys with him can die really easily, and so it may be worth flashing them down first.
There's no substitute for experience, and a little smart thinking. =)