In general, the following comps are what I've found somewhat productive at 100. However, I've found running instances (at 100) over the past several expansions somewhat frustrating and unproductive.

Tank - Pally or DK.

DPS - Hunters (BM or Survival)

Healer - Shammy or Priest

As Franswa notes 5 hunters is an interesting degenerative group (yeah, I wish Blizz would allow a premade 5 to queue without the role check). Another fun group that you can queue with is 3x Hunters and 2x Boomkins (with offspec tank and heals). The Boomkins casting a pair of Rejuvs as needed is handy.