10-19 rogues for killing noobs. basicly rogues are the best at this point for taking out "noobs" one after another nonstop. you can get lots of kills this way. only other reason to have one for 19 bg is sprint is the only travel form.
10-19 shaman for versitility. purge, healing, insane melee talents, very high defense, earthbind totem. those are their main benefits for 19. being able to get +5% block and +5% chance to crit at that level is amazing. being able to get rid of paladin blessings is a must for winning wsg. shaman are for winning wsg and defeating other twink classes, still good for farming honor but not quite the "noob farmer" as the rogue
20-29 priests are the best at 29 for support. they cant farm worth much in comparison to others but they can survive better than anyone else hands down. especially now that they dont have to go disc. to get 15 second shield cooldowns. when i say priests i mean TWINKED priests. non twinked they are annoying at best... twinked 29 they are impossible with any kind of partner helping. if you try to go for the priest, they can survive forever and you will be attacked by other enemies, if you try to go for their backup then the priest will heal and shield etc.
20-29 shaman again for overall ability. at 29 they become even more insane. as a fully twinked shaman who knows what they are doing, you will take out ANY twinked rogue 1 on 1 (engineering helps a bit). My current 29 wears the cobalt crusher (2hand mace, 3.2 speed) and i crit on cloth wearers for near 400 damage with a 17% chance to crit and +30% attack speed for 15 seconds after each crit lol... anyone under level 29 will be going down in less than a second when you crit+shock. i wont get too far into all the ways u can use shamans in 29 but heres a hit... ghost wolf, frost shock, PURGE, quick heals, DISPEL POISON. dispel poison btw should be bound to a nearby key and spammed whenever u are being attacked by a rogue

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i know i left hunters out, but really, they only have 1 specific job and function at both 19 and 29, to slow people down and run from melee classes. yes they are very annoying to run from and therefor help greatly to win many WSG games, but thats about it. If they are horde, then they need a shaman anyway to slow down the alliance paly flag carriers, otherwise blessing of freedom wins. they have OK damage but really just die a lot when targeted by twinks. my ideal wsg group would have 4 hunters yes, but not the ideal twink, just the ideal defense against flag runners.