Definitely a nice task!
My own list would leave no doubt as to the number one, whereas the others might shift their places depending on the mood, the moment etc.
Trying to strike a balance between "common" vintage (i.e. not so long ago and with a liquid enough market) and current production, and choosing a watch that one can comfortably wear everyday:

1. Omega Speedmaster Professional (the Moonwatch, handwound with current calibre): the cronograph if there is one, and the one to have if you have to settle for just one. Ref. 3570.50.00
2. Heuer Carrera 1964 re-edition handwound (same calibre as the Moonwatch btw) with white dial, ref. CS3113
3. Omega Speedmaster Professional Mark 4.5, ref. 176.0012 (but good luck with Omega Service for these!)
4. TAG Heuer Carrera Tachymetre (Val. 7750-based, date-only version) in the version with red chrono hands, ref. CV2014.BA0786
5. IWC Classic Pilot Chronograph, ref. 3717-04

I am not 100% sure of the prices of these in USD but they should make the budget.
Cheers,
Fabrizio