My preference would be druid. DK tanks are painful while gearing up as your mitigation (assuming you spec for it) is on a cooldown, meaning you can get into trouble if you're not careful. I've tanked heroics on my druid at 2800ish gearscore. Trying that on a DK is suicide. Druids won't compete with your DPS for gear and there are some really great druidish (druish? lol) tanking items available in the heroics that are easier to farm (i.e.: not HoS/HoL/ICC).