End result stats, like:
Total Major tournaments,Total weeks at the top, consecutive weeks at the top...etc..
Basically, the ultimate goal in Tennis is to reach number 1. That is what everyone is striving to. Wining matches, tournaments are means by which a player reaches the ultimate goal.
Secondary and last criteria is the Grand Slams total wins.
I think anything beyond that is great stats, but are WAY less important then the first two, and without the first two, one cannot be considered a goat. A player can be in 100 Consecutive finals, but not winning them and not reaching number 1 - will mean he will never be considered the greatest. Same with Titles won, no matter how many, if you are not at the top, Number 1, it is irrelevant in GOAT terms.