The only other series that I've seen that compares with this series is the Aus-India series in 2001. That series in India was literally one between two equals, and you couldn't really say that one team outplayed the other, which makes it all the more special. Significance-wise it was right up there as well, with India being the 'Final Frontier' and all and Aussie coming there on the back of 17 test wins.
But for the sheer quality of the performers, this series has got to be the best. This series featured four 90mph plus bowlers, a double-reverse swinging demon in Jones (the only one apart from Akram), PLUS a legendary fast bowler in McGrath, a legendary spinner in Warne, a legendary keeper in Gilchrist and a legendary all-rounder in Flintoff!! It was the one-on-one contests that these players featured in that made this series so special. The only thing missing from this series was an all-time great batsman (something which the India series had with Tendulkar, Waugh), but I don't think it detracted from it in a major way.