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.
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!
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.