Thief or Ranger??
Ranger is strongest at range, but we thieves make the best roamers/skirmishers.
7800 hours ingame, and counting.
Thieves are by far the most mobile of the and while Rangers have some great survivability tools I believe thieves played correctly win there too. Zerg warfare is where Rangers may shine however. Thieves really are scout/skirmisher type. If you want to rain hell on the zerg then ranger is probably better although a thief can do other things, sneaky things.
After the addition of anti-stealth traps in WvW, the answer should be obvious now.