On a galactic scale, I'd say Asari.
Aside from the obvious difference in skin tones and their "crests", Asari are physically/anatomically identical to humans (number of digits, face-shapes, overall body shape)
My reasoning comes from Javik. If you bring him on Priority:Thessia, when you first enter the Temple of Athame he comments on the relics stored there.
Fitting, since he was more or less THERE when that history happened.
He says that the Protheans saw "potential" in the Asari, which is why they were given the gift of biotics among other things.
For centuries, the Asari had a whole treasure trove of Prothean tech right in their backyard to study and learn from.
Who else had such "easy" access to such information?
Humanity.
Granted, it's located on Mars, but considering interstellar distances, it may as well be the corner market that's two minutes away on foot.
Perhaps it was tucked away on the next planet over because they saw our ancestors and realized that giving a species that weaponizes fire and rocks the means to harness dark energy would not be the wisest decision at the time, so they put it out of reach until we're civilized enough to handle it.
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Another thing to consider is skin tone itself.
Asari skin tones are generally blue.
Human skin tones are generally orange.
On a color wheel blue and orange are complimentary (opposite each other).
Take a look at the species' temperaments:
Asari (from an outside perspective) appear to take forever and a day to get things done. They plan decades in advance and often the people who see it take shape don't see the results because by the time it happens they've passed away.
Why get a small benefit now when in just 30 short years we'll obtain 20x the benefit?
Sure, they may be a threat now, but we'll let them simmer down for a decade or two.
Humans don't like to wait. We want it NOW and we're not going to wait!
We're not going to sit around waiting for procedure when we can just go and DO IT ourselves!
If you don't like it, tough, because we're not putting up with your crap.
Don't try to stop us, or we'll beat you up.
And don't push our friends around either, or we'll kick you into next week!
You see the difference?
The overall temperament of the two species couldn't be more opposite each other.
So to tie it all together, it could be that the Protheans wanted to "cover their bases" with the widest range of potential they could find, everything from the slow and deliberate Asari to the unstoppable, immovable Humans.