The most logical choice would be topping Laver's two CYGS seasons with three:
If someone chooses three CYGS - that player is also guaranteed at least 12 majors, at least 156 weeks at #1 and at least 3 year end #1s, though they're not record breaking by any means, those supplementary numbers would still make that person a candidate for GOAT.
If someone chose, say, individual records at any particular major - like 5 AOs or 8 FOs or 6 USOs - the supplementary achievements that go along with those records aren't very impressive - #1 ranking isn't guaranteed, other majors aren't guaranteed..
There is no guarantee three CYGSs would yield even one week as #1. Four slams give 8000 points. Nobody under the current ranking system has ever been #1 with as little as 8000 points. Currently Fed has more than 11000, and that with winning only one slam.