string[] folders = new[] {
	@"d:\projects\subfolder\1 word",
	@"d:\projects\subfolder\2 box",
	@"d:\projects\subfolder\10 apple",
	@"d:\projects\subfolder\100",
	@"d:\projects\subfolder\only words"
};

string largest = (
		from path in folders
		let m = Regex.Match(Path.GetFileName(path), @"(\d+)")
		let num = m.Success ? int.Parse(m.Groups[1].Value) : -1
		where m.Success
		orderby num descending
		select path
	).FirstOrDefault();