The "best" could be potential-wise.

Assuming the player was playing regularly and at top form, their knowledge of the mods/playstyles and consistency when Playing seriously would probably be the deciding factors.

So basically the potentially best player based on their performance when they were their strongest.
I'd keep in mind though that some players may seem amazing compared to the general standard of the certain time period that they were strongest at, but the standard may have risen since then.

I'm wording everything in such convoluted ways today...ahh