for _,player in pairs(player.GetAll()) do
    surface.SimpleText(player:GetRankText()) --NOTE: I know this probably wouldn't work because the syntax is wrong but I don't wanna look at the wiki atm.
end

--Note that 'self' in our PLAYER:GetRankText() function is whoever we pass. In this case 'player' from player.GetAll(). (I use player cause it looks nicer, most people would do for _,v in pairs.)