Well.. Jeff.. what a question - you are a cruel harbour master!

These are my thoughts.
Heuer have always been, in my eyes, a tool watch maker. Not in the sense of the modern term - aka 'big hard ass divers', but in watches that do a job, elegant or not.
I guess many will focus on the prettier models, but I'll meander about what is the Essence of Heuer.

The Monaco, being an unusual design is in the spirit of Heuer - going their own way. Sure it was a bit of a flop when launched.. but it's the icon now.
The Carrera is the obvious choice.. very much in fashion, and a beautifully clean design. The Chronosplit is arguably the most daring, but heart strings pull at a mech watch. The essence of Heuer is a difficult proposition. Is it the most notable model, or the model most notorious with a race driver.. hard to tell.

I'm going to buck the trend.. my vote goes for the Chronosplit.
I've never owned one, or handled one.. sure it's not pretty.. but it was/is a tool watch for the professional driver.
It sums up Heuer in a weird 'essence' - a drivers tool, breaking the mould, stcking one finger up to the rest.

Stewart
Heuerville