Transformer Oil

The question is too broad, more details are required such as transformer voltage and load rating, insulation type and class, location, previously used fluid, etc., it depends upon the manufacturer's specifications and what you find when you enter your question into Google. The "best" fluid in one situation may not be the "best" in another.

Agreed. Additionally this information can (generally) be found in the transformer oil supplier's/sellers marketing literature with a simple internet search.

As for me, I am quite partial to mineral oil for general purpose distribution transformer applications.

The best oil is what the manufacturer recommends. If you care about performance and warranty.

Otherwise, use EVOO.

Or, Retrofilling Oil