23mm vs 25mm - which is faster? is not a question that can be answered with just that info. Your weight is a big factor - a 25mm tire is faster than a 23mm tire for someone who's heavier without a doubt. The line between when 23mm would be faster based on weight - is debatable and is one I don't know about. But if you had a 240lb rider, they would be faster on a 25mm tire, all other things being properly adjusted.
18mm tires exist, but they're not used much, because they're to skinny and slower. Tires that are to fat are also slower. The fastest tire for you is one that's neither to wide nor to skinny.