Rome to Positano

"Best" is a subjective term so more info would be helpful. :O)

What month will you be traveling?

Are you coming from an airport and if so, what time is your plane scheduled to land?

How many people and how much luggage? Any mobility issues?

Price an important factor or not?

During the season the boats operate, the solution I see most often recommended is "fast" train from Rome to Salerno and ferry from there to Positano.

https://www.positano.com/en/ferry-schedule?from=salerno&to=positano#

If for some reason the seas are too rough for the ferries to dock in Positano, then SITA bus from Salerno. A private driver from Salerno would be the other option albeit the most expensive.

I agree that we need more information to answer. I'd also note that you don't get "advance" notice of whether the ferries will have bad weather. But taking the bus from Salerno to Positano requires a change in Amalfi. It's a single bus from Sorrento, but you have to change train lines in Naples and use the gritty Circumvesuviana to get to Sorrento. That fast train to Salerno, the Frecciarossa service, is much cheaper bought far in advance with non-changeable, non-refundable tickets.

I know that paragraph is hard to read. That's an indication of how hard it is to answer your simple question.