1 is not at all and 5 is completely.
When judging, you should not compare the game directly to other games in the contest. Instead, compare it to what you think the ideal example of that category would be.
If you think the game is on the same level as that ideal, it would be rated a 5.
If the game is completely missing any sense of that ideal, it would be rated a 1.
Most games will earn a rating somewhere in between.