For me, five stars means "I love this song," but there is a difference between songs that I love and songs that I consider my all-time favorites. And sometimes, I'd like a way to find these special songs without having to sort through tons of other top-rated songs.
For that purpose, I turn to the Comments field in iTunes. To see a track's Comments, select it in iTunes and choose File: Get Info or press Command-I. Click on the Info tab and you'll see the Comments field near the bottom. In it, you can write whatever you want, but in this case find a song you really love, select it, and enter #6stars in the Comments field. You aren't really rating the song six stars, but adding this comment will let you use that info to sort your favorite songs.
Next, from the File menu, choose New Smart Playlist, select Comment Contains, then type #6stars in the empty box. Make sure that the Live Updating option is checked, so any songs you tag in the future will be automatically added to that playlist. Now you have an up-to-date playlist of all the songs you've rated with six stars.
Script it
But who wants to have to enter a comment manually every time? That's where AppleScript can help. Start by opening Script Editor (located in Applications: AppleScript). In the new script window, write the following code: tell application "iTunes" to set comment of current track to "#6stars" then save the script (with default setting) with the name like "Rate 6 Stars" in your user folder/Library/iTunes/Scripts. (If you don't have a Scripts folder, create one in that location.) Then, whenever you listen to a song that you think belongs in the six-star echelon, click on the Script icon in the menu bar, and choose your script--it will add the comment automatically.