I don't think that would be a good question here, but I don't think it's mostly off topic. Read below the break for how to make it on topic, in my opinion.
It's totally fine to ask a question specific to one area here; "too localized" was removed as a reason for not being topical about five years ago. You're less likely to get a good answer the more localized your question is, of course, but it's totally fine to ask. You shouldn't necessarily ask a question specific to a small town, and questions tending to be too localized is a driver behind some of the "off topic" elements, but it's not a general reason for something being off topic, and it's never the sole reason for something being off topic.
It's also fine to ask questions about finances that are specific to parenting. It might not be the best place to get answers to financial questions - Personal Finance and Money is a great place to ask about 529 plans, for example, and has far more expert members in that regard - but if it's about Parenting, it's on topic here. Saving for your child's education is certainly a topic a Parent would be interested in; see our help site - it specifically says that "family finances" would be on topic (which this is a part of), just better asked on PF&Money.
Questions asking for "the best" are not explicitly opinion based, either, though certainly they tend toward that. There's no real difference between "How can I teach my child to read" and "What is the best way to teach my child to read"; we implicitly assume the "best" in the first question. Nobody's going to answer with the worst way to do something, in their opinion, after all! Parenting is a slightly squishier topic area site than most, where there isn't really a scientific answer to most questions (and even the scientific answers are often squishy), and it's understood that we can have multiple "good" answers to something. I would tend to avoid using the word "best" to avoid the appearance of asking for opinions, but it's not really a problem in and of itself.
Where this question goes awry is in asking for a product. This is specifically excluded in the same help page:
And some subjects are considered off-topic because they're not directly related to parenting,
shopping recommendations (too localized, and not timeless)
This is somewhere that the "too localized" term appears (though I wonder if it was added to that page before the "too localized" terminology was removed from the sites years ago?), but it's also appropriate to some of the questions we would otherwise get - the best place to get [something] is often so highly variable that it wouldn't ever be helpful. The best 529 plan in Illinois is different from Missouri is different from California; and thus the real problem with the localization is that we'd have different answers from everyone.
You're asking what state is best, which is slightly different, but still ends up in the same basic place - but instead of being too localized to a state, it's too localized to you. The best state is not generic! It's specific to where you live, to where your child might go to school, etc. - it's very specific, in a way that wouldn't ever help anyone else when asked just that way.
You'll also run into the other problem - the best plan is different tomorrow from today, perhaps, and next week, and next month. Stack Exchange doesn't really do all that well with questions that need updating; we struggle with that on Sports where we have questions "what is the world record fox X" when of course that can change frequently. Some sites permit that kind of question, but we don't, as it would be too hard to maintain the answers, and they'd be simply wrong at some point.
There's also another, probably overriding, reason for excluding product recommendation questions: spambots! As a former mod, I can tell you that any of our "product recommendation" questions that survived from the early years when closing questions was less common tend to be spam bot honeypots - random link-only answers to something years later, that's not usually very helpful, with no disclosed affiliation.
There is, however, a way to ask this kind of question in an on-topic manner. You shouldn't ask "what is the best 529 plan" or "what is the best state for a 529 plan", here or on PF&Money, but you are welcome to ask about what kind of characteristics make a good one. I'll agree with both Stephie and the help page and suggest you ask the question on PF&Money, as I think you'll get a better answer there; in fact, I'd first search the site, as this is not a new question topic by any means. I do, however, believe it would be "on topic" to ask this question here if you wished.
I am looking to to start a 529 plan for my child. What elements should I look for in a good 529 plan?
That's not very specific, of course, and so would have some challenges finding a good answer.
My first child was born last week, and I'm looking to start saving some money for her college fund. My state has a 529 plan option, and I'd like to invest in one. What should I look for in a 529 plan?
That's better, has the child's age in it, but still missing... hmm.
My first child was born last week, and I'm looking to start saving some money for her college fund. I'll contribute a small amount every year, planning to have enough for a state school by the time she graduates high school. I live in the US, and she doesn't have anyone else who is likely to contribute. I might live in a few places, as my job requires me to move around, so I'm okay looking at any state in the country. What should I look for in a 529 plan? What considerations might drive what state I pick?
There we go - now it has reasonable details to get good answers. You want a 529 plan that will be cool with a small budget, that will be around for a while, don't need to factor in grandparents/aunts and uncles/etc. - now someone could answer that question. It's a question that, while it doesn't have one correct answer, it is as close as anything in Parenting gets; it has a reasonable boundary, so it doesn't require a blog post to answer; and it's asking to teach you how to fish, not asking for the fishmonger's address.
Finally, if you do want to pop over to PF&Money, I would start with this question - it seems closest to your question.