The most difficult to achieve has got to be Tour + Olympic Gold + WC in a year. Only three riders have achieved this in a career that I know of (Zootemelk, Indurain and Ullrich).
For a start, you only get to even attempt this treble once every 4 years, for most riders that means at most 3 or 4 attempts.
It's not a like a May, July, September run like the GT treble; it's July (Tour), August (Olympics), late September (Worlds).
You begin with the hardest event. You rely on your trade team for the Tour, then possibly a different number of compatriots in each of the next two races, depending on how many riders your country has qualified in both Olympic and WC.
You need to be a versatile enough rider to win GTs and one-day races, assuming both Olympics and WC courses suit you.
Finally, how many Tour winners ride both the Olympics and the WC the same year they won the Tour?
The most prestigious treble? Maybe not, given the Olympic Games' status in pro cycling. Still, the Tour and WC... when was the last time that was achieved? LeMond in 1990, before that Roche ('87), Merckx in '74 & '71, Bobet in '54, Speicher in '33, Antonin Magne?