what kind of activities do you do in the miata? If you aren't big into autocross (don't need the MOST traction), there is NO better tire, IMO than the SO3 from Bridgestone. If you autocross, you wont find a better tire than Azenis...
If you want a decent tire, cheap, I found Toyo Proxis t1-s for $59 each online...
The toyos, so3, and azenis tend to be the top 3 tires ppl talk about.
Having driven the Azenis and now having the toyos, I'd say the SO3 gives the best all-around street/track performance.