Por favor...
Akkurat by Laurenz Brunner
http://www.lineto.com/
It depends which qualities of Helvetica you are considering as definitive.
For Preface, I kept the grotesque curve shape, angle of stress, and contrast, but changed many of the letterforms -- producing a different colour/rhythm.
So on the "curve" basis I would discount Proxima (too round) and Galaxie (too square), and their letterforms are also quite orthodox.
Knockout 31 makes an interesting comparison with Helvetica Regular, as does Maple -- they are both very "19th century grotesques". Going in the other direction, of minimizing the idiosyncracies of letterform, you end up with Frutiger.
Paralucent is a grotesque with a couple of novel letterforms, and some interesting details.
Chalet is interesting, but it would require taking the "a" from 1960 (IMO the a is the quintessential LC Helvetica character) and putting it in 1980.
Helvetica Alternatives and the accompanying newsletter offering a free weight of Preface.
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