California has been its own country as well, although not for a long time but it was when it was the Bear Flag Republic. That was a few centuries ago though, in a time period before electrical power grids, interstate highway systems, airports (and the FAA while we're at it), and other advancements in infrastructure that we have today. Also in a time period where even the most basic of commodities weren't outrageously expensive like they are today. Texas also had a similar stint as its own country a few centuries ago. I would say the top four-to-five states could be their own countries if they wanted to and be either modestly successful at it or widely successful at it. So CA, TX, FL, NY, and IL (though for NY and IL, if they become countries, they would have that Pearl River Delta sort of relationship with their suburbs across state lines, erm, I mean national lines).
The thing is that no matter how rich the government or how rich the state, when you make the transition into a country, you have to assume debt to get started. The loans will have to pay for a (potential) new currency, new regulations towards the business climate, border security, newly minted federal bureaucracies, so on.
Without a doubt, places like CA, TX, NY, FL, and IL have the largest economies in the country. All of them are comparable to other massive countries in GDP.
States of the United States versus the countries of the World in Gross Domestic Product (minimum threshold $200 Billion (for American states)):
01. United States: $16.8 Trillion
02. China: $9.240 Trillion
03. Japan: $4.901 Trillion
04. Germany: $3.634 Trillion
05. France: $2.735 Trillion
06. United Kingdom: $2.522 Trillion
07. Brazil: $2.245 Trillion
--. California: $2.202 Trillion
08. Russia: $2.097 Trillion
09. Italy: $2.071 Trillion
10. India: $1.876 Trillion
11. Canada: $1.825 Trillion
12. Australia: $1.560 Trillion
--. Texas: $1.532 Trillion
13. Spain: $1.358 Trillion
--. New York (state): $1.311 Trillion
14. South Korea: $1.304 Trillion
15. Mexico: $1.261 Trillion
16. Indonesia: $868.346 Billion
17. Turkey: $820.207 Billion
--. Florida: $800.492 Billion
18, Netherlands: $800.173 Billion
19. Saudi Arabia: $745.273 Billion
--. Illinois: $720.692 Billion
20. Switzerland: $650.782 Billion
--. Pennsylvania: $644.915 Billion
21. Argentina: $611.755 Billion
--. Ohio: $565.272 Billion
22. Sweden: $557.938 Billion
--. New Jersey: $543.071 Billion
23. Nigeria: $522.638 Billion
24. Poland: $517.543 Billion
25. Norway: $512.580 Billion
26. Belgium: $$508.116 Billion
--. North Carolina: $471.365 Billion
--. Virginia: $454.585 Billion
--. Georgia: $454.532 Billion
--. Massachusetts: $446.323 Billion
27. Venezuela: $438.284 Billion
--. Michigan: $432.573 Billion
28. Austria: $415.844 Billion
--. Washington (state): $408.049
29. Thailand: $ 387.252 Billion
30. United Arab Emirates: $383.799 Billion
31. Colombia: $378.148 Billion
32. Iran: $368.904 Billion
33. South Africa: $350.630 Billion
--. Maryland: $342.382 Billion
34. Denmark: $330.814 Billion
--. Indiana: $317.102 Billion
35. Malaysia: $312.435 Billion
--. Minnesota: $312.081 Billions
36. Singapore: $297.941 Billion
--. Colorado: $294.443 Billion
37. Israel: $291.375 Billion
--. Tennessee: $287.633 Billion
--. Wisconsin: $282.486 Billion
--. Arizona: $279.024 Billion
38. Chile: $277.199 Billion
--. Missouri: $276.345 Billion
39. Hong Kong: $274.013 Billion
40. Philippines: $272,017 Billion
41. Egypt: $271.973 Billion
42. Finland: $256.842 Billion
--. Louisiana: $253.576 Billion
--. Connecticut: $249.251 Billion
43. Greece: $241.721 Billion
44. Pakistan: $236.625 Billion
45. Kazakhstan: $224.415 Billion
46. Iraq: $222.879 Billion
47. Portugal: $219.962 Billion
--. Oregon: $219.590 Billion
48. Ireland: $217.816 Billion
49. Algeria: $210.183 Billion
50. Qatar: $202.450 Billion
51. Peru: $202.296 Billion
52. Czech Republic: $198.450 Billion
53. Romania: $189.638 Billion
54. Kuwait: $183.219 Billion
55. New Zealand: $182.594 Billion
56. Ukraine: $177.431 Billion
57. Vietnam: $171.392 Billion
The worst states would be places like Mississippi.
Mississippi Officially Abolishes Slavery, Ratifies 13th Amendment - ABC News
Just two years ago, Mississippi finally joined the rest of the world and abolished slavery officially. That is not to say they were practicing it all this time since the mid-1800s but they just now "remembered" to abolish it. You would have to think they were rebellious to the point that they didn't get it done following the Civil War, they held out, eventually forgot and are just now making the right moves. Welcome to the 21st century Mississippi.
The other four candidates would be other states like Mississippi. I'll leave it unnamed and assumed.