Depending on what you want to do with your computer, I'd say the main must-have package is the X server (and related packages). This will allow you to run any number of the available window managers and/or desktop environments, and is the base for most all GUI interaction you'll have with your computer.
If you're looking to utilize the sound hardware on your computer, I'd also highly recommend ALSA, and possibly PulseAudio. You'll have a very quiet computing experience without ALSA, as it runs your sound card and allows other programs to do so as well.
There are many other combinations of packages you might need, all depending on what you plan on doing with your system, but those two groups are pretty common to most desktop setups.