If you will be using your music only in Apple products, choose the AAC Encoder with "iTunes Plus" format, which is 256 kbps.
If you will be using your music also in other players, programs, and devices, choose MP3 Encoder at 256 kbps.
Of course, if you have any special requirements, e.g. audiophile-quality equipment, you can vary the above recommendations.
As you know, the above selections are made in the Import Settings dialog in Preferences > General.