First, look for insurance with a minimum benefit of 1M bath. Check if your chosen insurance plan has hospital treatment as "paid in full". This means that they will pay for surgery, anesthesia etc. in full up to the insurance limit. Then look at the prices of rooms and rooms in the ICU and how many nights they guarantee. 3000-4000 baths is the minimum for a good room. Then (most importantly) investigate how easily the insurer pays out claims.
I have inexpensive Aetna Opal 3 plan (750,000 bath max benefit). So far I had no claims, but I finished my insurance when I left for my country and they returned me unused premiums without any questions.
Cigna has a very similar Plan 4 (same premiums, room prices, 1M bath benefit)