On retail the best 5vs5 setup was 4 ele and a seperated healer but i guess thats irrelevant since 5s is here anyway dead.
I would say 1DK 4 ret is the best 5 man team overall.
High burst, decent offheals and lots of space for improvements.
The basic idea is to use the DK to catch opponents with deathgrip and Chains of Ice while the rets burst everything down that is in melee range.
Stun hammer is used to kill the opponent in the stun before he can even use his deffensives. Art of war procs are used to heal the team with instant flash heals up.
The setup has also a high dmg mitigation: Divine sacrifice + Hand of Sacrifice reduce the dmg taken by 80% for 10 seconds and this combination can be used 4 times on the DK (which should be the master of ur team) to make him basically unkillable for 40 seconds.
1 DK and 4 Ret is also a good PvE team as u can read here:
http://forum.warmane.com/showthread.php?t=364440