I put my vote in for DK (as I have a DK and 4 Shamans already downing heroics on farm).

DKs look the coolest (the abilities), have the coolest name, the best buff for an all shaman team (ebon plaguebringer), the best PvP utility, and the most DPS IMO (especially on boss fights) - I can pull 1600 DPS without trying in blues and heroic epics on my DK throughout an instance just using DnD, diseases, and Unholy Blight.