What kind of maze was Dom looking for in Inception?

What he was looking for was stated fairly precisely. I've added a few words here:

You have (a maximum) of two minutes to draw a maze that takes me (a minimum) of one minute to solve

The first two mazes are too easy, with a path that goes from one end of the maze to the other. He solves them very quickly. The concentric circle maze was much harder, requires more searching through the entire space to find the path through to the next layer. She even uses the notebook cover to create it, so she is not mentally constrained by the grid lines.

He doesn't care what the maze looks like, merely that she is creative enough to create a puzzle hard enough to quickly solve.

He needed someone that could create a dream environment complex enough that the projections find it too hard to easily navigate to find (and kill) the architect.

I do appreciate the fact that the circular maze looks unsolvable. I can't explain that other than this is a small mistake in the movie. I personally don't think there is more to this than the first two mazes were too easy.

Judging by the image posted by @iandotkelly there is one more thing that makes the desired maze stand out.

It is a maze within a maze within a maze. It is constructed of several layers of mazing, very similar to the dreams that the team has to navigate through later on.

So by designing the last maze, ariadne has used the same concept that they are planning to use later on in their dreams.

And if the maze is really unsolveable - at least for Dom - that might be a further hint of him not being able to escape the last layer of the maze he is actually in throughout the entire movie.