By today, Arya have been in the game for over a year since her infamous introduction.
Is she still the best?
I would say currently she still up there for the top units in Heroes, but not the best. Over a year stronger dragons and armor units took some her crown. She still tear apart most units in the game with her power specials. In the red sword category Karla and LegMarth can give her a run for me money.
Unique Specials:
In the past year only two other characters got a unique specials that being certain legendary (Fjorm, Ike, and Marth). So I believe the devs knew how tricky that is to implement and instead the route of giving more personal skills.
Surprise debut/new character on a separate banner:
The only other character than was introduced on a different banner was Rhajat for the voting gauntlet, unlike Ayra the trailer directly stated that she was on a different banner.
Overall while her launch was an experimentation of how powerful can a unit be at that time and how will the players react to it. I still hope that that was a one time thing.