10 of almost any caster class would be unstoppable in pvp, especially shamans, spriests or warlocks even I might post a QQ thread on the oboards if I got rolled by a 10man like that lol. As for pve
atleast 2 tanks, id go with either paladins or druids (likely paladins) for healers I would have to go with priests or resto shamans. atleast 2 shamans for their totems would be huge buffs to party and for CC+DPS mages/locks so hmm lets break it down
2 paladins (tank)
3 priest or 2 priest+1 shaman for healing
1-2 shamans (elemental) for dps
2 mages (would likely go fire for stronger dps)
2 warlocks for CC and dps
and maybe one spriest to round out the warlocks for dps.
3 healers are not a requirement at all but having 3 healers for 10man content turns it towards the trivial side of things and with the inherent difficulty of 10 boxing this safety net is something you will want.
Still cant imagine how painful leveling those up in a 10man group is going to be, you would have to grind on normal mobs and would taking a major exp hit for being in a raid.