It is kind of hard to judge as a whole, since different classes are good at different things. Ambushing, soloing, small skirmishes, defense, assaults, GvG, and zerging are all different beasts in nature, so analyzing which class would be best for all of them is difficult.
Anyway, were I to come up with a ranking system “as a whole”, I would put it at the following:
#1: Guardian. Makes a strong frontline, great group support, can solo anything that attacks him, good mobility and decent chase. Weakness include a lack of ranged weaponry and distanced AoE, so they aren’t good at defending or too massive of a zerg.
#2: Elementalist. They have high mobility, great support skills, good water fields for healing, a ton of AoE damage, good field control, good range. Problem is they have incredibly low survivability, so they are weak to ambushes, and when locked down they die quickly.
#3: Engineer. The engineer is like the lovechild of the guardian and the elementalist. The engineer is capable of stacking a lot of boons while being somewhat mobile, while also having decent support skills and massive ranged AoE damage. The engineer doesn’t have a weakness; there is so much build diversity within engineers that what would beat one build is stopped cold by another. Problem is, the engineer suffers from the “jack of all trades, master of none” syndrome, where everything they do, someone can do better. Still, it is always good to have on by your side.
#4: Warrior. The control abilities and the offensive power, combined with decent statistical bulk makes warriors another strong frontline class. The advantage they have over the guardian is that they have more viable ranged weaponry, and can defend towers as well as they can scrap it out in a zerg. The problem is, however, that warriors are disabled fairly easy, and most of their strongest moves require assistance from other classes to lay down.
#5: Necromancer. The necro provides a lot of control and AoE pressure, and also yields the highest statistical bulk out of any class. With a full life force bar, which is surprisingly easy in WvW, the necromancer has so much HP and power that recklessly engaging one means certain death. Their AoEs work as a good deterrent, capapble of hitting around corners, and can slowly lay waste to any opposing zerg. The weakness of the necromancer is that they are slow, often trailing behind the other team. They don’t have good chase or good disengage, so necromancers are like slow moving bastions on the field.
#6: Mesmer. Most of the mesmer’s abilities rely more on small engagements, so they have difficulty finding a place in WvW. Most mesmers are relegated to utility roles, like spamming focus #4, using veil when commanded, and portaling golems. Granted, the portal technique is really useful for re-taking keeps and getting golems in an area, but because of this you only need 1 or 2 mesmers per zerg. The mesmer is good at dueling, but without forced point control the mesmer doesn’t have enough speed to chase down opponents, with most of their disengage being from stealth.
#7: Thief. The thief’s role in WvW is usually relegated to ambushes an small skirmishes, and because of this the thief is rarely ever necessary. The thief can provide targeted kills and pulls well, as well as generally harassing solo running classes that are weak to ambushing. The thief’s greatest asset is stealth and blind support on small skirmishes; stealth allows the team to avoid larger zergs as well as have an immediate advantage in team vs. team fights. But otherwise, their short range and general squishyness make thieves ineffctive for GvG, zerging, and defending.
#8: Ranger. This is the hard part. You see, I gauge the classes based on how noticeable their performance in the WvW field is, or how well they perform while under my control. The biggest problem with rangers is that, for every ranger I’ve ever engaged, they’ve amounted to nothing greater than “I’m vaguely aware that someone is shooting me. Oh, it isn’t a warrior?”. That is to say, I don’t feel their field presence at all. Which is unfortunate, since I see a lot of rangers in WvW. They’re usually defending keeps by shooting at someone, or in a zerg tagging someone at maximum range. Other than ambient fire, I don’t see rangers performing that well at all. Rangers might as well not exist. I mean, I’ll miss using their pets as epidemic bait, but otherwise… they shoot at people? I"d inquire about their chase skills, but I’ve never had to run from a ranger.
I don’t have opinions. I only have facts I can’t adequately prove.
