Control Freak

Which profession allows the best control of the battlefield? Like who can basically "aggro control" the best? Or is it a group effort?

Tanking?
Elementalists, warriors to some extent, and sins if you want godmode easy.
EDIT: Also more things like 600 monks and 55 monks/necros.

not sure what you mean by best control of the battlefield, but the best aggro control would go to sins, due to shadowstepping. tanks can go w/a for the same effect.

Guild Wars doesn't have traditional MMO taunt aggro control methods. There are no skills that force enemies to attack you. Warriors and paragons have the highest armor in the game and have some skills that provide armor buffs, but should not spend time tanking. They, especially warriors, are damage dealing classes first and formost.

Now, that said, there are ways to control aggro, via positioning/bodyblocking and pulling only one group at a time. There are also plenty of ways to snare targets (rangers have cripple, elementalists have the water spell line).

What I guess what I meant was stuff like Cripple, Snares, KDs. Things of that nation.

Warriors for KD's from Earthshacker,or Ele for water magic snares such as Deap Freeze and Ice Spikes.

Ranger prime or secondary can also use crippling attacks also spirits that can slow and even traps but the traps are of limited use atm.

Necro's too have some ability to slow enemies

I think GW tries to give similar options to many classes some are better than others.
Since you can have any secondary class it really comes down to how you want to work it.

do you prefer getting into melee and then cripple enemies there or be a spellcaster type with much more magic power that can cast a lot of area effect spells to control the ground.

Ranger can also trap the line between the mobs and castors.

In PvE minions are very effective on tanking and keep mobs in the same place, so you can consider a Necromancer Minion Master (or Minion Bomber)

It is a team effort, as you asked about. Sure, certain classes can excel at tanking or snaring, but that doesn't mean that one person can keep aggro in control for a whole team. For example, if someone is tanking, and another person runs in too close to them before the aggro is set, then the aggro won't stay on the tank. Or, if someone is tanking and has the aggro controlled, and someone uses a spell like Firestorm which causes the AI to scatter, then aggro will be broken.

Having a full team of people that know HOW to manage aggro is far more important than 1 or 2 people with skills that help do so.

This is where your Water Ele comes in.

Tank uses Earthshaker.

But as you said, it is primarily a team effort, its more what the team doesnt do that becomes usefull, rather than what you team does. This is shown in your first example.
Agro controll is more about how you pull, kite and your positioning than skills and builds. The only agro controll skills I would consider in PbvE are Earthshaker, Deap Freeze, Ward Against Foes and an MM.

I would say that Monks are they are you lifeline keeping you alive.