It's both situational and depends greatly on your sort of controller.
The absolutely simplest way to control an enemy is to shove him, her, or it into the center of your melee team and prevent him from escaping while your melee strikers and defenders pound him into mush -- works much better against artillery and (some) controllers than soldiers and melee specialists, but there you have it.
The second simplest way to control an enemy is to immobolize or slow them, send them to a corner of the battlefield, and then forget about them while you fight the rest of their team while they're a man (or woman, or thing) down. Of course, this works much better against melee specilaists (and soliders), and not so much against artillery or controllers.
Barring hitting their weakness, the most effective ways to prevent an enemy from acting (aside from killing them) is to render them accross the board useless for a while -- knock them into another dimension, render them stunned or unconcious, turn them into stone, weaken them, daze them (for many enemies), or give them a huge penalty to hit, or put a huge wall between them and you. Obviously, you usually want to ignore enemies that have been so thoroughly neutered (except for unconcious ones, where it depends); they're not doing anything, and probably won't for a while, so it's best to fight the forces you've divided off (unless, of course, the enemies you've so removed are, say, elites or solos (without a giant penalty to saves), who are very likely to throw off your debuffs in a hurry. In that case, the debuffs are mostly keeping them from doing damage--but if they're also making them substantially more vulnerable, you're better off upgrading the "stunned" status into a "dead" status rather than ignoring them while they're vulnerable.
Finally, there's the method of controlling an enemy by simply making them hideously vulnerable--using tricks like automatic crits (or unconciousness which lets allies make coup de gras), granting huge bonuses to saves or huge penalties to defense (psions, I'm talking to you), applying vulnerabilities, etc. In this case, obviously, the controller is acting as a pseudo-leader -- and the correct action is isolate-and-kill.
The thing is, many controllers can have all or most of these concepts in their toolbox (in addition to things like bunch-and-blast, or the best-of-all-worlds stuff like area debuffs that lock down (daze, immobolize, stun, whatever) a whole battlefield, blaster controller tricks that force enemies to choose between "a lot of damage" and "even more damage", etc). So while which you should favor depends a lot on what controller you build, the real answer is very specific to the situation--keeping in mind the truisim that the best and most permanent status effect to inflict on a monster is "dead".
We're 9th level and my first combat w/the character had me cast 2 spells on a giant on the left that kept him slowed and out o combat for 3 rounds. Then I helped toss out things like fireball, burning hands and thunderwave on the enemies that were off to the right side of the courtyard. Our Warden and Rogue went off to handle the giant and 2 or 3 other bad guys while the Paladin and Cleric held off the guys I was helping to fry. It worked pretty well giving the Warden time to set himself up as super-quicksand 
