At your lvl I recommend Searing Gorge; Thorium Point + the dragon guy nearby give alot of quests that overlap and give you insane quest efficiency, and alot of them are kill x quests(though they sneak a few collection quests in); start/finish hinterlands, feralas, then ungoro about 50ish, then hit plaguelands at 52ish, hit silithus/winterspring between 56 and 58, and I personally recommend avoiding outland till 60 so you maximize your ability to kill with less downtime/dmg taken/surviving better + quest exp efficiency