Is there a competitive version of werewolf/mafia?
I am going to answer this assuming this is a convention, you are GMing the game, and there must be winners written in on a card at the end
If you are going to play 1 or 2 games you should pick the winners yourself.  Just decide who did the best job.  Did someone manage to lie to the group really well, or did anyone lead the group, get them past the chaos, and lynch for victory?  If you do not feel comfortable picking, let the players vote, but let them only pick from the winning team.

If you are going to have 5+ games you could consider a point system.  Maybe just giving a point to everyone on the winning team.  Whatever you do, do not let the players know the scoring  If  people know how the game is scored they will game the system.  If this happens it will most likely ruin the game for everyone, (and probably let the wolves cakewalk their way to victory in later games).

Players will quickly think “he is winning, lets lynch him just in case”.

Again, if there are ties, I recommend just picking winners yourself; let the players vote on who they thought was most helpful.