The best race is entirely dependent on what you want to do with your Dragonknight.
For a tank, I would say Imperial. Their racials are really strong for it.
For a dps with a heavy stamina build, Redguard would be my choice. Racials are built for it.
For a dps with a heavy magicka build, this might sound weird, but Breton may be the best. Maximum magicka increase and reduced cost of magicka abilities is quite appealing for a player who wants to go heavy Dragonknight abilities/Desto Staff. Dragonknight does not get a whole lot of options to choose from to restore magicka, so every little bit will help.
For a dps with a balanced build, Dunmer provides a nice balance with their racials.
For a healing build, Breton all the way.
This is my Dragonknight tank
build. Gives me a variety of options depending on the situation.