Ferry's Remix was in 1999. Funnily enough, that version exists mainly thanks to Tony McGuinness (Above & Beyond):
"...I worked at Warners on the original, aborted release of William Orbit’s “Pieces In A Modern Style” in 1995, and played the unreleased CD all the time: the first track was Barbers Adagio. I’d come home from clubbing and stick it on, and thought it might make a good trance record. My brother and I started mucking about with it in my studio, and with some other classical Adagios I had Midi files for, including Albinoni. After William became famous with Madonna, I started talking to him about releasing the album properly and maybe getting a dance mix of Barber’s Adagio done. He was very keen. This was before I met Jono and Paavo and so in the end I sent the midi files to Ferry Corsten and he did the mix to my brief: Gouryella but with classical music. He did a great job. I suggested Albinoni to William as another track for his album but he didn’t like it, so I thought I’d do it myself some day. It took a while to get round to it."
Skip Raiders also made a vocal version of Adagio For Strings in 1999 called 'Another Day', which was slightly less popular than the Ferry remix overall, but became Oakenfold's go-to version and was used in the film Kevin & Perry Go Large.
And Tiesto was playing his version as far back as 2003, it was the opener to his first 'In Concert' that year. Not sure about In Concert II, never watched that one. It was fully released in early 2004.