If you're looking for opinions, here's mine...
My faves, in order of submission:
#34, first one:
#59
#120
#134, red background
#152
I also like the concept of the puzzle, but I didn't list it because something about the execution just didn't grab me.
The ones I listed could use some tweaking here and there, but are great.
The rubik's cube is the most eye-catching, and a recognizable concept that applies to the subject. To me this could be the winner with a little work, because it immediately conveys the subject matter.
The card maze is also a standout for a similar reason, and it ties more into poker specifically rather than just cards because of the chip stack.
The color-mapped spade seems the most complete to me and quite original, but a bit more abstract than the rubik's cube, or card maze.
Most of the entries were great, some quite beautiful, but these seemed to be the ones that would make the book stand out more compared to other poker books.
I had my own idea, but don't have the time to execute it.
I wish everyone the best of luck with the contest!