Group 2. Sk Cleric Enc Mage Wiz Bst
This would be my suggestion. It basically covers all the bases with great abilities and versatility.
Sk: Good tank, great agro holding abilities, able to FD, self heal (lifetaps), DD spells, and the list goes on..
Cleric: Great buffs, Great Healing, Rez....
Enc: Great CC, Mana regen buffs, Runes, Haste, Slows, Debuffs, Pet...
Mage: Great pet, Summoned gear (for pets), Good Nukes, Mana rods, CoH...
Wizard: Great Nukes, Ports, Great Nukes...
Beast: Great pet, Good melee, Good nukes/Dots, Good Buffs, Healing, PARAGON, can also slow and minor debuff...
The question is: at what levels are you planning on running these LDons? My 105 SK can Solo swarm ANY LDon and my 100 War can Molo swarm with his merc cleric, so unless you are planning on running them at level appropriate, it doesn't really matter what the group make-up is. (with a group of 85 heroics (War, Clr, Shm, Bst, Mage, Necro) I was able to pull groups of 10-20 mobs and have no real issues with them unless for some reason the ones in the back started summoning people away from the group (I hate mob warping/bouncing..lol) then I
MIGHT have a toon die.