Depends on who you can get to. If the eladrin ranger is the only one who can teleport to the boss while everyone else is fighting soldiers and brutes, then it would be a very bad idea for the ranger to attack the boss.
My general answer will be, whatever enemy you can focus fire on and take down fast, is the enemy to attack. I've had fights go both ways. Sometimes the "boss" is considerably squishier than the elite brute in the front line. And may surrender when his brute falls. Sometimes, a gang of bandits may split when you take their leader down, so it's hard to make a generalized strategy of what's best. The more mobile a party is, the more options they will have for focusing fire on the biggest threats.
My experience in a lot of fights has been that the artillery end up being some of the last creatures to die, despite the fact that they deal a lot of damage (and as such are a big threat), just because they are harder to get to. But on the bright side, combat ends really fast once they are the only ones left on the board since they go down quick (or run away/surrender, or I just narrate they've been neutralized and move along).