What luxury watch should I buy?

There are so many great answers here: Rolex, for all of the reasons already stated, Omega Speedmaster (for the same reasons mentioned by another) at the lower-to-mid range of the price spectrum and certainly if you can find a Jaeger LeCoultre or a Patek Phillipe for under 20K, they are investments in both elegance and long-term value.

I would add A. Lange and Sohne and IWC as well, at the higher end. Beautiful watches.

Since no one has mentioned Breitling, some of them are very handsome watches but somewhat heavy on the masculne sports and aeronautic. I would put Breitling side-by-side with Rolex and Omega.

I wear Panerai, like them and they range from $6000 to above $25, 000. The look also varies from sports-masculine to dress-elegant (Radiomir). Bill Clinton wears Panerai but it is not a particularly discreet looking watch.

Many watches cross-over well from business elegant to casual wear. Find one that you like and consider buying pre-owned from a reputable dealer. You'll save a few thousand dollars for an item that may be been gently worn for a few years. My opinion.