http://https://www.project1999.com/forums/showthread.php?t=229828
But my general experience (most active before hybrid xp penalty patch) was: counting all tank classes meaning anyone with the taunt skill, not counting level 60's, there was rarely many Rangers or Paladins to be found.
By far the most common was warrior, but if you count anyone who "tanked" in casual Xp groups it was monks by a landslide.
Checking that link's population chart the least common however would be a gnome warrior of which there was only 1 online at that time.