What is the single most important financial habit to develop?

Consistently spend less than you earn and invest the difference automatically. Almost every good financial outcome flows from this one habit, and almost every bad one flows from breaking it. Your savings rate – the share of income you keep and invest – predicts wealth far better than your income or your stock-picking ever will, because it controls both how fast you build assets and how little you need to live on. The way to make it stick is automation: route a fixed percentage to investing the day you get paid, before lifestyle has a chance to claim it, and raise that percentage with every income bump. Everything else (budgeting tactics, fund choices, tax tricks) is optimization on top of this foundation. Master this one thing and the rest gets much easier.