It seems that we're coming to the idea that the following three factors are not necessarily synonymous:
1) Difficulty of a title
2) Desirability of a title (to a player)
3) Historical legacy of a title (towards a player's "greatness legacy")
A Grand Slam is the highest in all three, with Wimbledon ranking highest followed by the US Open and Roland Garros shortly behind, and then the Australian Open. After Grand Slams you have the WTF as a clear second fiddle, but also above the rest of tournaments. Where it gets tricky is Masters vs. Olympics. I'd say the Masters is more difficult but perhaps a tad less desirable and with similar historical weight. After that there's a big drop to ATP 500 and 250. Davis Cup seems almost in the category of doubles - entirely irrelevant to a player's resume.
So maybe, in terms of relative value of historical greatness, it is something like this, with all numbers being a precentage of a Grand Slam:
100 Grand Slam
40 WTF
20 Masters, Olympics
5 ATP 500
3 ATP 250
Or something like that. If we want to break it down further with Slams, we'd have to talk about years - and then it becomes really messy.