It’s important that you buy at least two single-breasted, two-piece suits. The two-button look is a longtime business standard, but the three-button look has been making several comebacks. It will often depend on your height, as taller gentlemen has a better chance to look good in a three-button suit.
While a black suit always works, a navy blue one is stylistically a better option, as it’ll help you subtly stand out from the professional crowd. Similarly, a dark gray suit (with or without pinstripes) provides a number of shirt-and-tie combos.
Even if you’ve picked up a couple of suits, you probably need at least one blazer too. As always, stick to brown, gray, black, and navy. Avoid fabrics like corduroy and velvet, as well as tweed patterns. While these can be cool and trendy, they can be a bit too informal for the traditional office — only wear them if you work is less formal.