Generally according to Forum guidelines we dont talk about watches for their investment values (although there may be in the very rarest of cicumstances ok). I expect that for around $20k, your choices will be in the simple calatrava range. There are many many very nice watches in this range of watches:
Automatic: 5127 or maybe the predecesor 5107 is still available. The main difference is a slightly diff case and 5127 is larger
Manual: 5119 and 5196. Both are great. I think 5196 in PT is kind of unique and is relatively slightly harder to find
New Nautilus range: Sportier watches and quite a few in Steel, so more afordable too. These are popular with the younger generation I think....
As to investment value, you will be told we dont know what is best and that this is not why we are here. But what ppl will say is that generally the more expensive the PP and the longer a reference has been discontinued the more chance that it is more in demand.
My advice: the best investment in a watch is not to buy it as investment. The only way to ensure you end up with a small fortune from buying watches is to start with a big fortune!!