If you were going to purchase one watch, and have that be your only watch for the rest of your life, it had better be a Rolex Submariner. Yes, I know we all know about the Submariner, and yes, I know it's been talked about ad nauseum, but that's partly the reason why I'm talking about it today. You see, there are four categories of watches, and this is the only watch I can think of that comfortably sits in all of them. Those categories are dress watches, sporting watches, everyday watches, and investment watches.
Thanks to James Bond, the Submariner can be counted as an acceptable dress watch, it's simple black and silver design is unobtrusive and simple enough to look sleek (especially when compared to today's gaudy monstrosities). You could wear it to a board meeting, a black tie gala, and anything in between. It is just as at home with a tuxedo shaking hands with CEOs and foreign dignitaries as it is underwater fighting off evil henchmen.
Which brings me to it's original heritage as a sporting watch. It was created to be the professional diver's watch, built to withstand the pressures and hazards of things like clearance diving (when you dismantle underwater explosives, no joke), submarine welding, recovery diving, etc. There's a reason is the standard issue watch to those whose expertise is life threatening underwater activities. With a heritage like that, I'm pretty sure it can handle hunting, hiking, camping, fishing, swimming, sky diving and anything else you can throw at it. It's caliber has been refined over the years to be even more shock resistant and durable under almost any conditions.
Because of it's existence in the two former categories, it has become the ultimate formidable everyday watch. It will obviously have no problem on the beach, in the rain or snow and looks stylish with every single outfit you can imagine. Which is also probably why it is the most copied watch design in the history of watches. It's a fantastic value for those reasons alone, even if you don't scuba dive or venture into the wilderness. It is also made of proprietary 904L steel which is tougher than your average stainless steel and more resistant to nicks and scratches.
Like any high end timepiece, if you take care of it and aren't reckless when wearing it, it will undoubtedly over time increase in value. Like a Ferrari, when you buy it new, there are plenty of them running about, however over time many are crashed or neglected by shoddy owners, eventually making the ones in excellent condition worth an incredible amount of money. Indeed old Rolex Submariners in good condition can easily be worth anywhere from two to four times the amount of a new one. This makes it a great investment watch over time.
This has always been my ideal watch, and probably always will be. The most idyllic and collectible submariners are the rare 5513 milsub, the 1680 red sub, and the super rare 6358 James Bond sub. And as a commenter below pointed out, you should definitely buy them used unless you have to buy it new from the store. If you buy one used, make sure it has full documentation and has been inspected and does not need a servicing, alhtough this should all be standard from any reputable seller. Eventually, I will undoubtedly buy one, but until then, it remains relegated to the Wish List.