Collar shape should follow your face and neck — a spread collar suits a narrower face, a point collar works for most builds, and a button-down reads more casual by default. Fabric weight matters more than most people think: a lighter poplin drapes cleaner under a blazer, while an oxford holds its shape better on its own.
Fit is where most shirts fail before they even leave the store. The shoulder seam should sit at the edge of your shoulder, not past it, and there should be enough room through the chest to move without pulling at the buttons. Everything else — cuff style, pocket or no pocket — is preference, not fundamentals.