Are we talking intro level classes? If that is the case, and it certainly sounds like it is, I would think any four year school would do. A community college or an online class probably wouldn't impress Harvard too much.
On the other hand, I went to a large university and there were around 400 people in each of my intro to micro/macro econ classes, (I got the highest grade in one of them by the way), but I believe that the "economies of scale" at such an institution preclude what you might call "particularly rigorous".
First of all, with your background, I would only take the classes, if they are prerequisites for the programs you are considering.
Next, I would either go to a school based on its location OR to one that seems to have a professor that is doing impressive research.
My guess is that you would prefer the second option.
So, if that is the case, research the individuals at a few institutions OR read a few scholarly journals, and contact some of them and say, "I think you do great, interesting work and I want to be your research assistant for a semester or two."
Impress them with your passion and smarts, get a great letter of recommendation from them, and go on to a great grad program.