Which speakers to isolate first?

Is your system set up and have you figured out the optimum positions for your speakers and listening seat?

If not, do that first, then work on isolation, diffusion, absorption, diffraction.

If you can access the space, see if you can strengthen the floor. 

Put the subs on springs like Nobsound etc.

@mvanwoert 

I would start with the subs.

At first sight that might seem odd advice because it's known that our ears are far less sensitive to bass distortion than any found in the midrange.

On the other hand in real world terms if you are likely to have audible distortion then it's more likely to be found in the low bass than anywhere else. This is due to the greater movement and work being asked of the drivers and the cabinet.

You should definitely experiment with isolation because going from a concrete to a wooden floor is a fairly dramatic switch in support.

Equipment placed on wooden floors seems to benefit far more from some form of isolation too.

A simple method might be to initially use some cheap foam/rubber under each speaker.

Perhaps some experiment with placement too if your situation allows. I recently watched this video which shed some further light on the complex issue of boundary reflections.

Perhaps it might be of some help to you.

Whichever is more important to you.  midrange is my favorite so I would start there.  The Nobosound spring suggestion is a good one if your speakers aren't too heavy.  Put those on the set that you can't afford to isolate right away and they will help a log for just a little money. 

Jery