Use get_players and then loop through all the players to find out which one has the highest get_user_frags. If two or more players are tied used cs_get_user_deaths as a tie-breaker.
Also bear in mind that the first player on the scoreboard isn't always the best one, a guy with 40 kills and 40 deaths will be 1st and a guy with 38 kills and 2 deaths will be 2nd.