The info below is a bit out of date as it refers to the beta. If you happen to be looking for an up-to-date reference for your Dragonknight, I suggest this eso guide . It includes some killer advice on leveling up, builds, skill rotation, and gold farming.
Each race in The Elder Scrolls Online will have racial passive bonuses, Ultimate abilities and EXP bonuses that may be important when creating your character. While we don’t know every bonus of each of the races yet, we do know the bonuses for four of the races. The bonuses themselves are quite varied and not necessarily groundbreaking, but when maximizing your dragonknight abilities, any bit often helps.
Note: The Elder Scrolls Online is still in beta, the below racial bonus information is subject to change.
If you are planning to create a Dragonknight, the following races have these bonuses that will help out a Dragonknight the most:
Khajiit
Robust (Passive): Increased health regeneration during battle.
Carnage (Passive): Increased critical rating for melee attacks by 10 and damage done by successful critical hits by 5%.
Argonian
Argonian Resistance (Passive): +26 to poison and disease resist.
Quick to Mend (Passive): Increased incoming heals from all sources.
Histskin (Ultimate): Increased Health regeneration for a short period.
Nord
Battle Cry (Ultimate): Battle shout that inspires you and your allies, increasing damage dealt.
Two-Handed Weapon Expertise (Passive): Increases skill EXP for this skill line.
Robust (Passive): Increased health regeneration during battle.
Resist Frost (Passive): Increased resistance to cold damage.
Rugged (Passive): Increased natural Armor rating.
Dark Elf
Ancestral Guardian (Ultimate): Allows the Dark Elf to summon a guardian shade that will protect him/her for a limited time.
Ambidexterity (Passive): Increased skill EXP for the dual wield skill line.
Dynamic (Passive): Increased maximum Magicka and Stamina.
Resist Flame: Increased resistance to Fire damage.
As you can see, the Khajiit and Dark Elf bonuses apply more toward DPS Dragonknights. The Dark Elf bonuses, especially, are geared toward dual wielding. The Nord and Argonian bonuses are geared more toward tanking Dragonknights. The Nord bonuses are especially solid for Dragonknights who choose to use a two-handed sword.
As recommended in elderscrollslevelingguide.com, when it comes to choosing the best race for a Dragonknight who wishes to pursue tanking for their leveling in teso, based on this information alone, it looks like the Argonian bonuses or Nord bonuses may come out on top. The extra bonus to healing received as an Argonian may be extremely important when it comes to endgame, but the increased armor rating of a Nord may easily make up for it.
For DPS Dragoknights, it will depend on the type of weapon you want to specialize in. For two-handed DK, Nord is still extremely strong. For dual-wielders, both Khajiit and Dark Elf have some nice bonuses. The Carnage bonus for Khajiit may be extremely powerful.
The info above is a bit out of date as it refers to the beta. If you happen to be looking for an up-to-date reference for your Dragonknight, I suggest this walkthrough . It includes some killer advice on leveling up, builds, skill rotation, and gold farming.