Is there an optimum orbit for a Hipparcos/Gaia-like parallax observatory
There is several reasons why the L2 point is an optimum point for the satellite. 

First you have a stable orbit with low gravitational perturbation and which is not too far away. So, you won't have a huge travel time. And the parallax is good for what the satellite is supposed to do.
Secondly in a Lagrange point you can easily point out the same point. In orbit, you will have some grey area. 
Eventually, I think you are doing a mistake when you say : "Assuming no cost". In my opinion, if we are not sending a satellite like Gaia around another planet, it is because we want to avoid risks and sending the satellite, around Mars for example, adds a ton of risk. 

That's why we use the L2 point to place a satellite in regards to all the parameters. But, you can choose another location if you want to maximize something in particular.