I think there is a fairly objective way to quantify the importance of tournaments, even choosing the 4 most important event in a year.
Tournaments should be given „prestige points” (from 0 to 4) and “draw quality points” (from 0 to 4). So the maximum number of points is 8.
Here is my proposition for the „prestige points”:
4: GS
3: year-ending championships (and the WCT finals)
2: 9 premium events (called ATP 1000 today), DC Final
1: other important events (called ATP 500 today)
,0: smaller events
The only difficulty is to assign the premium events before 1990, but in this way we only have to take into account prestige and tradition, without looking at the entry field
Here is my proposition for the „draw quality points”:
It is based on the top 8 participants (current weekly ATP rankings, before 1984 interpolated monthly rankings)
The participation of the No 1 player is worth 4 p, No 2: 3 p, No 3-4: 2 p, No 5-8: 1 p (max 15 points)
So the „draw quality points” for tournaments:
4: 11-15 p
3: 7-10 p
2: 3-6 p
1: 1-2 p
0: 0 p
Here are some points for tournaments (I consider the first 4 the most important):
1968: WIM 8, USA 8, FRA 7, PSW 6, AUS 4,
1969: WIM 8, USA 8, FRA 8, AUS 7, PSW 6,
1970: WIM 8, USA 8, Sydney Dunlop 6, PSW 6, FRA 5, AUS 7, Masters 5,
1971: WIM 8, USA 7, FRA 6, AUS 8, Masters 5, WCT 7,
1972: WIM 6, USA 8, FRA 6, WCT 6, AUS 6, Masters 6, PSW 6,
1973: WIM 6, USA 8, FRA 8, WCT 6, AUS 6, Masters 7,
1974: WIM 8, USA 8, FRA 6, WCT 7, AUS 6, Masters 5,
1975: WIM 8, USA 8, FRA 6, WCT 5, AUS 7, Masters 6,
1976: WIM 8, USA 8, FRA 7, WCT 6, AUS 5, Masters 5, Philadelphia 5,
1977: WIM 8, USA 8, FRA 6, Masters 7, AUS 5 (both), WCT 5, Philadelphia 6,
1978: WIM 8, USA 8, FRA 7, Masters 6, AUS 5, WCT 6, Philadelphia 6,
1979: WIM 8, USA 8, FRA 8, Masters 7, AUS 5, WCT 7,
1980: WIM 8, USA 8, FRA 8, Masters 7, AUS 6, WCT 5,
1981: WIM 8, USA 8, FRA 8, Masters 7, AUS 5, WCT 5,
1982: WIM 7, USA 8, FRA 8, Masters 7, AUS 4, WCT 6,
1983: WIM 8, USA 8, FRA 8, Masters 7, AUS 7, WCT 6,
1984: WIM 8, USA 8, FRA 8, Masters 7, AUS 6, WCT 6,
1985: WIM 8, USA 8, FRA 8, Masters 7, AUS 8, WCT 7,
1986: WIM 8, USA 8, FRA 8, Masters 7, no AUS, WCT 6,
1987: WIM 8, USA 8, FRA 8, AUS 8, Masters 7, WCT 6,
1988: WIM 8, USA 8, FRA 8, AUS 8, Masters 7, WCT 5,
1989: WIM 8, USA 8, FRA 8, AUS 8, Masters 7, WCT 7,
So in 1968 I choose PSW (Pacific Southwest) as the 4th important event. In 1970 the Sydney Dunlop and the PSW are 3rd and 4th. The problem here is comparing the Dunlop (2+4=6) and the AUS (4+3=7) events in Sydney. The former had better field, but the latter had more prestige. From 1972-76 the WCT, from 1977-1986 the Masters, for the sake of consistency. But looking at the points, in some years other events could also be the 4th event (like Masters 1973, AUS 75, AUS 1985).
What do you think?
This system is crude, but it gives an opportunity to evaluate events during the whole course of tennis history, using the same criteria. I am ready for the years 1977-2012, and slowly working my way backwards, but I can give you the obtained points for any tournaments you are particularly interested.
Perhaps giving the same weight to prestige as the draw quality is not fair. In this way the AUS Open (and many amateur slams) already gets 4 points, even without a Top8 participant.