All depends on what you need it for.. Ultimate got more features than other two but it costs more as well.
Windows 7 Ultimate offers a few extra features on top of all other editions. In this regard, Microsoft has chosen to highlight BitLocker, an encryption technology for the computers running the platform but also for removable and portable storage devices. In addition, Windows 7 Ultimate is the only version of the OS, with the exception of Enterprise, that can easily jump from one Multilingual User Interface (MUI) to another, seamlessly jumping among 35 supported languages.
Other than the ability to log into AD, Home Premium and Ultimate are the same OS. Ultimate has more utilities, but the OS isn't different.
It all depends on what you need it for. Windows 7 ultimate has a few more features that the other pkgs do not. But if you don't need them it's useless.
As for me the simplest version of Windows 7 is the best one. Nothing lees that the most necessary tools and gadgets!