London to Oxford

By train, but where in London are you coming from? You can depart Paddington or Marylebone. Time depends on the train route, but 1hr approx from Paddington, longer from Marylebone.

Price depends on when you're traveling and ticket type (ie peak/offpeak, whether you want an anytime ticket etc), which you haven't told us.

The station is a short walk into the centre.

On the other hand the 'Oxford Tube' bus terminates in the City Centre at Gloucester Green Bus Station, is cheaper and with fixed fares (£15 single, £22.50 return) and runs 24/7 up to every 12 minutes in the peak- https://www.oxfordtube.com/

Whereas the train depends on time of day with cheaper fares being dynamically priced.

The longer journey time of around 2 hours each way is largely due to London traffic before getting to the Westway A4/M4- https://tiscon-maps-stagecoachbus.s3.amazonaws.com/Timetables/Oxford_Timetables/Oxford_Tube_current.pdf

And, of course, no standing, which you may sometimes get on the train.

Depending where you are staying Victoria may also be more convenient.

So "best" is subjective, not an absolute. But the bus is very popular, hence the size of bus used and the frequency, indicating that many people are willing to trade off frequency and cost against travel time.

it comes down to where in London you are starting rom, and where and when you are going in Oxford