Hort & Jansa: the Best Move

I loved that book back in the 80's.  I would guess 1600 and up over the board ratings.

However, I understand that the new edition was cheaply printed according to reviews on Amazon. 

I have the German version (Der beste Zug). If you solve all 200 problems correctly, you get 933 points. In the preface the authors mention that if one scores 900, his/her Ingo is around 100 which is about 2040 ELO. I would say it is below Aagaard’s Calculation and Imagination in Chess.

I've used this book personally and with students.  My take on the level is very different.  As an example, the russian work "The Manual of Chess Combinations - Chess School 3" self states its level at 2000 FIDE.  Which seems about right to me.

Jansa/Hort provide difficult problems from their own games.  They were both strong GMs.  I would estimate the level based not only on this, but also on experience working with the material at a level >2350 FIDE 

For reference, my old (still current) USCF rating is a little over 2200, from the 70s.  

Most agree that the level is harder than Dvoretsky / Aagaard.

Aagaard Calculation 2300 fide and up. Hort and Jansa 1800 and up. Russian  MOCC 1900 and up. Imagination in Chess (Everyman) 1700 and up. Perfect Your Chess 1900 and up. Just my wild estimate and so with other reviews. I am not rated player but a pure chess addict passionate dedicated maniac. 

As a USCF life master currently both 2220ish in fide and USCF, I can tel you I have done the first 100 problems with about a 50% score. These positions are from actual gm games. The level is at least 2350 or higher.