How to find the player with the highest score?

A useful pattern you'll need is this loop:

local bestScore
local bestPlayer --player with best score
local players = game.Players:GetPlayers()
local player
for i = 1, #players do
    player = players[i]
    local score = --[[fill this in here to get the score of player ]]
    --ex: local score = player:FindFirstChild("leaderstats") and player.leaderstats:FindFirstChild("Score") and player.leaderstats.Score.Value or 0
    if not bestScore or score > bestScore then
        bestScore = score
        bestPlayer = player
    --Note that if score == bestScore then you might have a tie. You could keep track of multiple "bestPlayer"s in a table if you wanted to handle this.
    end
end
--At this point, bestPlayer will refer to the player with the most points

bestScore starts out as accepting any value (in the first iteration), but after that it is only allowed to grow. You only update bestPlayer when you find a new best score.

Note that though this solution can be made to work with finding the order of people (ex who is in first, second, third, etc), a better solution is to use table.sort (but this is a separate question).