On all of my characters nbs are by far the most killed class since patch 1.6 and I also get killed by nbs the most, so I assume that nbs are the most played class in PvP atm.
Back at PC launch, without a doubt dragon knights. The dragon knight population was nearly double the population of all 3 other classes combined.
It evened out a bit, but I think even today it's a very close matchup between dragon knights and nightblades for the most played class. Nightblades were moderately common at pc launch, but died off quickly when people realized how massively underpowered and buggy the class was, most of those probably transitioned into playing dragon knights (which were the absolute kings of the game back then). I think the nightblade population has started to rise again as improvements to melee weapons, stamina builds, and nightblade abilities have been added to the game, as they're the logical go-to class for pure stamina builds.
I'd say templars and sorcerors are roughly even, maybe leaning more in favor of sorcerors being more common.