Well, first of all, let's just say that all artists are human and also suffer from bad days when nothing seems to go right. (Remember too that we only see what the artist has allowed out of his/her studio or to be bought or sold by a dealer).
Let's put together a group of artists irrespective of when they lived and try to identify some common criteria, which may define their greatness: Michelangelo, Henry Moore, Rembrandt, Van Gogh, Picasso, Constable and South African artist Irma Stern.
Create your own checklist of criterion against each artist and score each out of ten. Even though it is entirely subjective you will find a certain logic pervades and scores may be similar for each parameter, here are our scores for Rembrandt and Picasso '.
So they both score 44 with different scores for each criteria. Which is best? - Not a good question - they are different but both great and may be bought by the same or different collectors. It is also true that art can and will defy rational analysis - just because it is ART! Remember that for both artists their early work sold for very little and Picasso often sold or left sketches for food or for a good lunch.
Who is the best artist?
Wrong question. What do you like? Remember it is completely personal.
Art mediums are the materials used by the artists to create a body of work. Oil paint, watercolour, acrylic paint and ink are popular media types, so too are a number of dry art mediums including pastel, charcoal, pencil and graphite.
The base material on which the pigment is applied is also considered an art medium. Canvas art, framed art and works on textiles, cardboard, wood, Perspex, paper, terracotta and a host of other materials are regularly used.