If you play a lot of vehicles then here is a very easy matrice to consider;
1. Does the unit have 36" range+movement capability or better?
If Yes, it might be a threat, if No you can probably dance outside its threat for a while.
2. Does the unit generate 3+ shots?
If Yes it might be a threat, if No then with our built in invulnerable save we can probably endure it for a while.
3. Does the unit's attack have a Str of 5+?
If Yes it might be a threat, if No then our toughness ought to soften any potential attacks.
If you ask the three questions you will get a very good idea of what is a threat to your army, and what is not. Let's consider it very quickly;
5 Space Marines with bolters.
1. No
2. No
3. No
Answer = not a threat.
5 Devastators w. 4 Heavy Bolters
1. Yes.
2. Yes
3. Yes
Answer = threat
5 Space Marines w. 1 Lascannon
1. Yes
2. No.
3. No
Answer = very minor threat
Predator with 4 Lascannons
1. Yes
2. Yes
3. Yes
Answer = threat
Then, once you have your threats understood, you can ask yourself the final target priority questions;
4. Can I kill it/make it useless this turn?
5. How much of my army will it take?
6. How exposed will it leave me?
These last three cannot be answered easily because they start to depend on what your army is, what is still alive in you and your opponent's force, and what the mission parameters and board layout are. But those are what you should be thinking of as you assess the threats on the table.