For me, the ideal one-watch collection should meet these requirements:
1. Water resistant at least 100 mtr with screw down crown.
2. Three hander, without or , with a date , because sometimes I will need to see the date. But it has to be done right, no 4.30 position, no subdial with small needle. And no cyclop.
3. It has to be a watch that can be worn on bracelet. An integrated bracelet is better.
4. Medium sized, in between 35 to 40 mm. Thickness max at 12 mm.
5. Legible dial with decent lume on hands and hour markers.
6. Alternating finish in brushed and polished.
7. Robust movement, anti magnetic, has good shock absorption.
So far, in my humble collection, there are only 2 watches that meet those criterions : the Rolex explorer 36 and IWC Ingenieur. I can't decide which one should be one-watch collection.
The other watches that I don't have yet is :
Moser Streamliner center second (but prefer in blue dial).
PP Aquanaut (if they can update the bracelet design, which now is ugly IMHO).