If you have one suit, make it a navy suit. It’s conservative, dressy, and easy to accessorize for festivity; and if you ever find the need to dress-up jeans or khakis, just wear the suit jacket by itself and voila – a navy blazer!
This is the argument frequently used to justify a navy “first suit.” It’s not wrong, but not every navy suit can be worn “broken down.”
The cardinal rule of navy sport coats is that they should absolutely not look like a lone suit jacket. After all, any sport jacket-involving ensemble should appear coherent, rather than giving the impression that you meant to wear a suit but forgot the pants. Since casual pants tends to be textured, sometimes even “rugged,” the jacket should likewise be rougher, at minimum matte. For a navy suit to be look good when as an odd jacket, it must pass for sport coat. In other word, it shouldn’t look much like a suit.
Subsequently, some have drafted suggestions accordingly that first-time suit buyers find an especially casual blue suit – essentially a blazer with “normal” buttons and matching pants – as to enable splitting. I disagree. The problem with this approach is that the best “blazer suits” aren’t great “first suits,” let alone “only suits.”
Take the Havana navy suit from Suitsupply, for example. While the fact that it has matching jackets and pants as well as buttons that aren’t metal makes it a “suit” by technicality, it is nonetheless casual. The soft shoulder construction, shorter jacket length, and patch pockets – all the features which give the suit the casual twist that helps it look “casual” as a sport coat – make it inappropriate for conservative attire (think wedding, funeral, interview, board room meeting, etc.). If you only own one suit, then I assume it is because you have no reason to wear it other than those rare occasions requiring formality. The supposed versatility of the Havana would thus be completely irrelevant.
Instead, my suggestion would be to seek the middle ground between blazer and suit. Get a suit in a solid, medium shade of navy (i.e., won’t pass for black, but getting there) with a slight texture (hopsack is good; avoid sharkskin because it “sheens” and looks out of place without matching pants). As for styling, look for a classic two-button with vents and flap pockets, and make sure that the shoulder padding is light and not overly stuffed.
This configuration would give you a decently versatile suit, acceptable for formal occasions; with a jacket that can pass as a blazer when worn with jeans, khakis, or other slacks. Now, if I had a choice between one amazing suit like this and a decent suit and a blazer, at the same price, I’d go for the suit and blazer. But, if you have a budget and could only choose one, consider buying a navy suit that is ready to be split.
Cover: A navy pinstripe suit from SuitSupply (source). Don’t ever wear this as a sportcoat.