For me it's Knight, it may not be the "strongest" in the PvP circles by and large (mages/ranged kill us way too easy seems like) but I don't like to PvP anyways.
I like it because I like to PvE and go dungeon and be able to take a hit and get up front in the battle. (I play a Tank Hybrid)
It also lets me be more flexible to help guild members with their dungeon, ladder and whatnot, makes me feel good to be able to protect the "squishie" guys. XD
All it all, it's up to your preference of playing. If you like to solo dungeons, play something more on the defense side like a knight, if you wanna PvP, probably should go with a mage, rogue, or a blood priest.
Rangers I don't know much about to be honest.