Travel from Siena to Rome

People have all kinds of preference there is really no best. If you are not driving there is a bus that goes direct to Rome otherwise you take the train with a couple of connections.

Here are the options
https://www.rome2rio.com/s/Siena/Rome

If you're traveling by train, it is probably easiest to go Siena to Florence and the Florence to Rome.

I'd also suggest the train - we did train to Florence from Rome and then a rental car to Sienna but it isn't something I'd recommend - parking in the city can be confusing and a number of areas are off limits to non-local traffic.

You can't get from Siena to Rome on public transport. Well of course that it is an overstatement, but it is darn inconvenient. It makes more sense to do Siena and other Tuscan sites first, then drop the car coming into Florence, spend what time you have for Florence then take the fast train to Rome which only take 90 minutes.

Four of us took the Flixbus from Rome to Sienna. It was an hour faster without connections. Cheaper and tickets were very easy to get online. Downside is shabby bus station and trains are usually better Nice bus with WiFi though.

My 75 year-old mother-in-law and I took the bus from Rome to Siena last March, based on recommendations on this forum. It was simple and easy. The taxi drivers in Rome know where to drop you off for the buses at Rome Tiburtina station. We got off at the Siena Piazza Stazione and took a short taxi ride to our agriturismo outside the city walls.
Don't be afraid of the bus system. It was cheap and easy. I booked my tickets in advance online. By the way (if it matters), I have NEVER taken a Greyhound or other long-distance bus in my life. So I felt nervous, but there was no reason to be nervous. It was great--clean, comfortable, and free wifi.