I am using "The Educational Plan" primarily because I get a meaningful income tax deduction in my state of domicile.
https://www.theeducationplan.com/OFI529/
Ever mindful of commissions and other costs I bought it directly, not through an advisor, although one can buy the plan through an advisor. I found it very easy to do over the internet.
Vanguard, Schwab and Fidelity must have plans which may have lower costs. If I were domiciled in a state not providing a state income tax deduction for enrolling in this particular plan I would probably go with Vanguard.