I'd vote for our state's plan - Connecticut.
https://www.aboutchet.com/It recently moved to Fidelity and uses the low cost fidelity index funds. Till last year it was managed by TIAA-CREF.
I also have money in the New York plan. I am a non-resident taxpayer in NY State, and it allowed for deductions for contributions till last year. Now that there is no tax implication, I am going to fund a new CHET account instead of putting the money in the new york plan.
There are many other options. Now a days almost all of them are pretty good. So I think it just comes down to your convenience. Maybe check if any state's plan is run through your preferred broker and use that?