Fine Artists use a wide variety of media but many people consider that Oil Paintings are the Gold Standard. This is for a variety of reasons.
The majority of paintings in any art gallery exhibiting Old Masters, will be oils. In Europe oils were first used in the 11th century but not widely accepted until two centuries later.
The Rate of spring, By Pol Ledent
The medium allows a wider variety of techniques than any other material. Textures from thin washes to heavy impasto are achieved with ease. Laying one colour on top of another can hide the base colour or allow the lower layer to shine through. Opaque, transparent, and translucent painting are all possible, and the variety of surface texture is huge.
In a similar way, canvas is seen as the Gold Standard as the surface to paint on, although some artists use wood panels, aluminium and a variety of other long-lasting materials. The bonus of oils, apart from its flexibility of technique is how long an oil painting will remain as fresh as the day it was painted.
Venice Fruit and Veg, By Nevine Hunt
One characteristic which is both a bonus and a drawback is how slow-drying an oil painting is. It is however, part of the reason Old Masters survive today.
You can find thousands of Oil Paintings here https://www.artgallery.co.uk/listing/category/oil