Any music genre, any time. For me it would have to be to 1966 and the start of Blues-Rock and meet Eric Clapton and The John Mayall Blues Breakers at the recording of the 'Beano' album. The year and album that heralded the change-over from Blues to the more guitar inspired era of Blues-Rock and the musical birth of the Rock God Guitarists such as Clapton, Page, Beck and Hendrix.
I would want to go back to 1976 and meet Steve Harris from Iron Maiden and see how ambitious he was regarding the future of the band.
He was touted as a future star player for West Ham as well at the time and I would like to see how he felt about giving that up to concentrate on the band when nobody would have known how successful the band would go onto be.
Simply put... I would want to go back and meet BB King.
I really don't think I would need to explain why, except maybe to say just one word: HISTORY!