There's been a bit of discussion recently regarding the viability of certain classes' condi damage. I thought it would be useful to have a look at a few of the more optimal builds and compare them.

Methodology
These tests were performed in the Heart of the Mists against an invincible golem. All classes were geared with a Rabid amulet in order to minimise power damage while still providing the  crit chance that many condis rely on. The builds are taken from metabattle, Death and Taxes' site or occasionally a video build guide. A DPS meter was used, and the standard rotation  performed for ~90s or 5 'kills' of the golem, whichever was longer. This is long enough for the average dps to level out. The numbers obtained are therefore 'real' and were double checked  against the time to kill a golem 5 times to ensure accuracy. Several variants of each build were run, and I chose the highest result for each class.
Only "useless" sigils have been used, as many builds use sigils that are not available in PvP.

DPS results
Reaper ~5000
Engineer ~4800
Ranger ~4400
Berserker ~3800
Guardian ~3200

Aside from the Engineer, the reaper was easily the highest DPS class, largely thanks to its ability to quickly combo a field 12 times. I believe the engineer will beat the reaper if they are playing perfectly, but unfortunately I am not good enough to manage this myself. I relied on a simplified rotation by Nike that provides a good way to use the best skills off cooldown, which is still very good. Of note is that during the test my ping jumped a bit and resulted in the dps dropping to around 4400 due to delay in proper kit switching. This lends some credence to my belief that between individual skill and the more chaotic nature of raids only very good engineers will be able to out-damage an average-skill reaper
(up for debate, of course).
Guardians seem particularly poor here. Their condi relies almost entirely upon whirling wrath, and they are poorly equipped - to get the most out of their burns, they have to use every utility slot for damage, they have a number of traits where two good options compete, and a lot of their damage relies on symbols, which could provide unwanted light fields at the target (though they can time their whirls around this). They also lack cc on condi weapons, and their best option (binding blades) is prone to latching on to seekers.

Combo fields merit a bit of discussion here. Fire is pretty much the best field to have, allowing you to blast it for might and whirl in it for burning. This actually means if another player can provide a field, the Reaper's DPS will be even higher than here, as they lack their own fire field. A single ranger or berserker is able to provide near-constant fire field uptime if set up appropriately.
The reaper's own combo field does high enough damage on its own to merit it being dropped into a fire field, even if it means losing the burning bolts.
The best fields to whirl in or fire through are: Fire > Ethereal > Poison

Flaws
The stats provided by a PvP amulet are obviously lower than will be obtained through ascended gear, food, etc. so these numbers will all be on the low end. Viper's gear is also not
available in PvP, but this deficiency should effect most of the classes in a similar way. This is supposed to provide a rough idea of how classes sit in comparison to each other, the numbers themselves shouldn't matter.
As each class has been taken on its own, this doesn't look at the abilities of other classes to boost one another. For example by stacking might or providing auras. There are so many different ways these could manifest I thought it better for my sanity to just leave it be.

The point of this is not to definitively say which the best condi class to use is, but rather that I believe any class has something to offer in the condition damage role.