The work of critics in literature, music, or theater is the lowest of all professions.
Literal
Literature, music or theater [toward-に対する] critics [possessive-の] work [topic-は], all work [among-のうちで] most lowly work is.
~に対する means 'toward' or 'regarding,' modifying 批評家. ~のうちで marks the superlative scope: 'among all jobs.' 最も下等な uses 最も (most) + 下等 (lowly/base) for a harsh superlative judgment. A provocative literary opinion.