This is a double question. Most popular with regards rider numbers?
Well from a Queensland point of view the period from mid nineties to mid 2000s would be it. Queensland won it's very first Australian MX Championship (Mick Cook 125 class 1995). Then for approximately ten years won at least one Championship each year. This was the period of Mick, Andrew McFarlane, Peter Melton, Michael Byrnes, Troy Carrol and then later Daniel Reardon. It is when Chad Reed rose to prominence. It also saw a round of the World MX Championship run in Australia for three years 2000 to 2002. The Thumper Nats also was at it's best during that period. Again in Queensland the eight round Shell Advance/Sunshine State MX series averaged between 350 to 400 riders at each round. Also Supercross still meant something in that period.
Most popular with regards spectator numbers?
The MR MX series in the early eighties would cover that. Also Supercross rose to prominences in that period. Of course the prominent riders in this period were Gunter, Gall, Vandenberg, Leisk, Bell, Moates, Ellis Dack, etc. If I have missed some riders I apologise.