the one you want.
This is a really hard question almost like asking which one of your children you like more. And again even that question is abstract for many.
If you liken this to a journey or a love story you can see how you can let the passage of time guide the opportunity.
So I guess what I am say is try and get the best you can in that moment in time and enjoy it while you contemplate the next one. You may find that the one you have satisfies you and the lust may die down and the love story of getting old together will continue. Like marriage you need to play the field while you are young. Sometimes you want sporty, some time classic and others just exclusive or tied to a particular event or occasion. Once you have leaved the dream you can choose to settle down a little. However unlike a marriage you can choose to settle down with a few of these beauties and no one will be upset. Except maybe the few you sold along the way.
Sure I would love to have a 21 or a nice preV but those are scarce and there are a lot of other model like the 249 or the Zero that are hard to get but still within the realm of possibilities.
Just hop on the Panerai train it has many stops along the way to meet new watches and build new relationships.
Happy hunting.
PS. I can't spew all this dribble above and give my wish watch - So stealing from Nicolas list - 5218-201/A, 202/A, and 301/A