For the past decade in online poker two HUDs have dominated, PokerTracker and Hold'em Manager. They are the Coke and Pepsi of the poker HUD world.
I have written about both of them extensively on this website in the past. And as a long time online professional poker player myself, I have always used one of them.
Always having HUD data available on my opponents has helped me win tens of thousands of dollars more at the poker tables. If you take online poker seriously, then using a HUD is a complete no-brainer.