Most encounters have a lynchpin. Take it out and the rest of the fight is a cakewalk.
I've always considered the weakest foes to be the lynchpin, not the strongest NPC (with the rare exception that if the strongest falls, the rest flee) or even the NPC controller.
For example, say that you have 3 standard foes and an elite. The group can focus fire and kill one standard foe in one round, or the elite in two rounds. The elite does 20% more damage (this is high, but the elite probably has at least one rider encounter power) than a standard, say 12 points instead of 10.
Attacking the elite first:
round 1) foes do 42 damage
round 2) foes do 42 damage
round 3) foes do 30 damage
round 4) foes do 20 damage
round 5) foes do 10 damage
Attacking the standards first:
round 1) foes do 42 damage
round 2) foes do 32 damage
round 3) foes do 22 damage
round 4) foes do 12 damage
round 5) foes do 12 damage
In the first case, the NPCs get 14 attacks in for 144 damage.
In the second case, the NPCs get 11 attacks in for 120 damage. The attacks on average are stronger, but there are fewer of them.
It typically takes a longer time to take out stronger monsters such that the monsters get in more attacks total within the encounter if the PCs ignore the weaker foes. And with fewer attack rolls per encounter, the odds of NPCs getting one or more criticals on the PCs in an encounter drops as well.
There are exceptions to every rule (e.g. a foe who can attack multiple PCs every round), but I've found that the weaker foes fall quicker and hence give the PCs an action economy advantage over their foes faster.
As a player, I almost always try to take the toughest foes out last.
Using this as the primary strategy, Neuter and Destroy tends to work even better than Isolate and Kill as long as the toughest foes are the ones being Neutered.
Using the example above and assuming that the Neutered foe can be neutered for 2 rounds starting in round 1:
Attacking the neutered elite first:
round 1) foes do 30 damage
round 2) foes do 30 damage
round 3) foes do 30 damage
round 4) foes do 20 damage
round 5) foes do 10 damage
Attacking the standards first:
round 1) foes do 30 damage
round 2) foes do 20 damage
round 3) foes do 22 damage
round 4) foes do 12 damage
round 5) foes do 12 damage
12 attacks 120 damage vs. 9 attacks 96 damage.
This doesn't always work, but it's a fairly good tactical rule of thumb.