Guitar Hero III is easily the best of the series with a song Line Up which is hard to rival!
Gameplay:
Guitar Hero III has improved on pretty much everything the previous editions offered which included elements such as Boss Battles, Encores, Co-Op etc. as well as offering endless amounts of items/characters/songs etc. to unlock with the money you earnt from playing which I think has worked in the games advantage and combined with all the gameplay originally conceived with the original easily makes this the best edition to date.
Sound:
The sound of the game would have to be judged and determined by the players tastes but I think it offers a wide range from the Genres of both Rock and Metal and is not biased to one time frame or particular sub-genre. I found myself never getting bored and constantly challenged by the different game types and difficulties and never got sick of any of the songs as I thought it was an outstanding collection this time round which easily puts the other games to shame and with all the songs available to be unlocked or downloaded it offers the best experience bar none.
I found this edition of Guitar Hero to be the best in the series with a song line up that could be rivalled by none and with all the new features would honestly recommend that everyone gives this one a try