Picasso was one of the best artists of the 20th century
Picasso fue uno de los mejores artistas de siglo veinte.
Here fue is better because we are speaking of a "definite" past, we are stating a past "concluded" fact, from the point of view of the present.
Pablo Picasso was a painter and sculptor.
Here we have less context, hence the decision "fue/era" is ambiguous. If, say, that phrase is the beginning of a Wikipedia-like article, then the sense is roughly the same as before, hence fue is the correct alternative.
Suppose instead that we are in the middle of a narrative. We are writing about the life of Max Jacob and we assume that the reader does not necessarily knows who Picasso was.
In 1900 Max Jacob met Picasso in Paris. Pablo Picasso was a painter and sculptor that had just come from Spain, and Max helped him with the French language.
A correct translation would be:
En 1900 Max Jacob conoció a Picasso en París. Pablo Picasso era un pintor y escultor que recién había llegado desde España, y Max lo ayudó con el idioma francés.
Here we must use era because we are speaking of past fact (the occupation of Picasso) but from the point of view of that same past (i.e., relative to that perspective, the event "Picasso was a painter" is a "present continuous" event, something that "was happening" and which was not concluded at that time).
All this is not specific to the "ser/estar" verb, but rather to the general indefinite/imperfect past tenses in Spanish; see eg here.
I want to use era because we still call him an artist today, and some may say be still is one of the best artists of the 20th century
To emphasize that aspect, you would not use era: you would simply use the present (just as you wrote in English) "Picasso es uno de los mejores artistas", or to be more precise and avoid confusion: "Picasso es considerado uno de los mejores artistas" (he is regarded as one of the best artists...) or "Picasso está entre los mejores artistas" (he is among the best artists...) In these later cases, it would be wrong to use the past tense, because the verb (to be) does not refer to Picasso's past life or ocuppation (who was he? what did he do?) but to his present position.