Depth of field the best ever - Worldwide

Hannah, Smith, Howerton, LaPorte, Ward, Croft, Reid, Pomeroy, Ellis, Weinert, DeCoster, Jonnson, Weil, Wolsink, Lackey, Malherbe, Thorpe and a dozen others I left out from 1975-1983 would like to have a word.  Mid-late 70's Trans-AMA series did exactly what you desire.

I loved that era but there's never been as many good riders as there are right now 

If someone had the time, it would be interesting to see how many championships in any class each rider had.  
Maybe mid 80’s ( Hannah, Bailey, Ward, Johnson, Glover, Barnett, Lackey, Lechien) but then again you had the 125/250/500 class back then. 

I started a project looking into something similar awhile ago.  I was looking just at the 250(Premier) SX races.  I was only looking at how good the riders were at that point in their careers for each race, using an Elo rating.  (So future wins don't help, and if you hadn't been good recently it doesn't help.)  I started in 1974 and went through 1989.  (I keep meaning to finish the project.)  At that point, the best race was 1978 Seattle.  In that race, the 10 best active racers were: Jimmy Ellis, Pierre Karsmakers, Bob Hannah, Marty Smith, Tony DiStefano, Kent Howerton, Jim Weinert, Jim Pomeroy, Gaylon Mosier and Danny Laporte.  

Karsmakers came into that race as the 2nd-best active racer, he finished 19th and never raced SX again.

The next-best was 1981 LA Coliseum:  Hannah, Mike Bell, Howerton, Broc Glover, Mark Barnett, Jim Gibson, Donnie Hansen, Chuck Sun, Steve Wise and Marth Smith.

One of the worst was 1989 LA Coliseum.  The top 10 that day were: Jeff Ward, Ron Lechien, Jeff Stanton, Ron Tichenor, George Holland, Shaun Kalos, Doug Dubach, Todd DeHoop, Keith Bowen and Micky Dymond.