In my opinion, and this is backed up by our own guru BK, the one gen Rolex to buy is anything that has a 3185 or 3186 movement in it. That would be an Explorer II or a GMT Master II. You pick the flavor.
In terms of the bang for the buck, these 3185 and 3186 movements are stellar works of achievement and to get one of these as a gen is a no brainer. Any other model that has a time + date complication is hard to really compare with a well regulated and serviced ETA movement. They both tell the time and they both tell the date. Yes, obviously fit and finish and all of that is separate, but just in terms of the movement itself, you aren't getting anything more unique.
The 3185 and 3186 movements sell for the same price as other Rolex movements and yet the GMT complication is brilliant. It's remarkable and no other watch offers the same performance with a GMT. Start with one of those and then build a collection out from there.
Just my opinion.