The goal of a suit -- for a man who only owns one -- is to be impeccable and appropriate for the few situations it's needed in.
If your looking to have only one suit, a go to suit or this is your first suit purchases there a certain aspects you should be aware of. You might have just started a job, which requires you to wear a suit. Or your looking for a suit to wear to funerals, suit that you are going to wear to graduations, weddings or any big events. In this case, you want to go with a solid navy or a solid charcoal suit. My personal preference would still be a solid navy. My feeling is that if you change the tie and shirt on a daily basis then one could get away with one navy suit as easily as a charcoal. I believe either charcoal or navy would be great for most events. Second consider the style, either a two or three buttons single-breasted fits 99% of occasions. Many do find that double-breasted suits tend to be a little more formal. It’s not nearly as versatile. That’s why Dapperness recommends going with a single-breasted suit. Also if you only own one suit, it should undoubtedly be a solid fabric (no stripes or checks). It should be 100% wool and medium weight (so you can wear it comfortably in a wide variety of climates and temperatures).
Versatility matters more than unique flair for a one-suit man. If you own 1 or 100 always make sure fit is perfect, so it is very very important to get it tailored!