Disclaimer: I'd like to preface this by saying that, practically speaking, the races are on even footing as an Elementalist. None of the current meta builds for WvW include using any racial skills since our class-specific utilities are too great to pass up. However, in the spirit of the question, it is possible to justify a "best" race. Just don't expect it to grant you better performance than any other race.
In order to answer which race is the best race, we have to first consider what your race actually does. Your race in GW2 affects a few things:
Your physical appearance
Racial-locked skins
The first half of your personal story
Racial skills
Certain NPC conversation options
Also, we have to consider what we mean by "best." My assumption is that "best" means "most effective," and "most effective" includes damage output, survivability, and mobility. As such, only some of the above elements apply:
Your physical appearance
Racial skills
Finally, let's assume that by saying "best," we are agreeing to use the WvW meta builds of Dagger/Dagger roaming or Staff zerging. Anything else is out of the realm of "best."
Physical Appearance
The part of your physical appearance that is relevant to your success in PvP would be your visibility. Low visibility improves your survivability as players will attack you less frequently as they do not notice you. It also grants you more opportunity to scout without being spotted. Also, smaller characters' animations are harder to see or interpret, meaning your opponents won't notice the telegraphs of your abilities as easily.
Shorter characters have less vertical visibility. Thinner characters have less horizontal visibility. At a far distance, your nameplate is not visible and players will have to recognize you purely by your silhouette. As such, you want to be non-noticeable. Technically, your character's skin color and armor color could help them camoflauge. However, a player can select their settings to show team colors which overrides your armor coloring. So, we won't include colors.
Asura are small in stature. They are the smallest race, and therefore the least visible.
Charr are the second tallest race. They are also the widest race and very visible.
Norns are the tallest race. They will easily stand out against a clear background.
Humans and Sylvari are the same height, but larger than Asura.
In terms of visibility, Asura is the best choice.
Racial Skills
So, physically, Asura is the best race. However, visibility doesn't matter much once you've been spotted. You will need to fight or escape! As such, racial skills might improve your effectiveness.
Asuran Racial Skills
Asuran utility skills grant an Elementalist access to Confusion, Retaliation, Poison, Weakness, and a long-ranged Daze. The elite skills give you a summonable golem or the Power Suit.
Charr Racial Skills
Charr utility skills have one that grants extra Fury and Might, an extra 15-second cooldown evade with damage attached to it, and a bleeding trap. The elite skills can fire a ground-targeted AoE, conjure a weapon, or summon minions.
Norn Racial Skills
Norn utility skills grant access to an extra bleed and an summoned (immobile, but powerful) minion. The elite skills are all shape-shifting skills that grant new abilities with knockback, knockdown, cripple, daze, life-steal, and bleeds, however no one form has them all.
Human Racial Skills
Human has a healing skill that is a 30-second cooldown casted burst heal. The utility skills include a condition removal and a random condition + boon applier. The elite skills include a transformation, a minion summon, and an unblockable chill + poison applier.
Sylvari Racial Skills
Sylvari also has a healing skill that provides rooted regeneration near a targeted location. The utility skills include an immobilize and summoned turrets. The elite skills include one skill that summons multiple turrets, and others that summon minions.
Skill Usefulness
Many of these skills are essentially useless to Elementalists because we can already do them, regardless of race.
Summoning Minions: All skills that summon minions pale in comparison to Glyph of Elementals and Glyph of Lesser Elementals because the Glyphs allow you to situationally select an Elemental based on your attunement. All other summons pigeonhole you into a single selection.
Conditions: Some skills give you access to conditions. As Dagger/Dagger and staff, you will already have access to Weakness, Fury (from Arcane Fury, one point into Arcane), Chill, and Bleeding. Dagger/Dagger also has access to Immobilize (they have access to other conditions as well, these are just the ones the racial skills provide). Dagger/Dagger typically uses Sigil of Doom to apply Poison passively and continuously during combat.
Weapon conjures: Elementalists are the weapon-conjuring class. Weapon conjuring skills are useful for other classes, but not Elementalists.
Heals: Since you cannot have the racial heals alongside the Elementalist-specific heals, I'd argue they are useless. The two most used heals for meta builds are Signet of Restoration and Ether Renewal. The Human 1-second cast heal and the Sylvari range-limited Regeneration can't match up to the effectiveness of these.
Knockback: Dagger/Dagger and Staff both have access to knockback.
So, what is left?
Asura: 3 stacks of Confusion on a 30-second cooldown and 1200 range Daze. Also, transformation elite.
Charr: Extra evade on a 15-second cooldown.
Norn: Transformation elites.
Human: Transformation elite.
Sylvari: Turrets.
Considering so many races have transformation elites, let's take a look at that. You only get one elite, so to use a transformation elite means that you don't use Glyph of Elementals or Fiery Greatsword. Roaming Dagger/Dagger almost requires the Greatsword for mobility as Elementalists have a criminally low amount of access to Swiftness without traiting for it and not nearly same mobility as Thieves or Warriors with the Greatsword. And Staff Elementalists don't really get in-your-face, meaning that going into a transformation would force them to go into melee and then back to range after the transformation is over.
In my opinion, transformation elites are a wasted elite slot (unless transforming into a Tornado, but even that is weaker now than it once was).
At this point, we are left with just the following abilities:
Asura: 3 stacks of Confusion on a 30-second cooldown and 1200-range Daze.
Charr: Extra evade on a 15-second cooldown.
Sylvari: Turrets.
In my opinion, none of these are particularly great. However, if we are strictly comparing the races to each other, it would appear that Asura has more viable Utility options than the other races.
So, who's the best?
Asura is the best in terms of visibility and Utility. Therefore, it is the best race for Elementalist.
