oneriest

/ˈoʊ.nər.i.ɪst/
adjective
  1. Most burdensome or difficult to deal with; superlative form of onerous.
    • Among all the chores, cleaning the gutters is the oneriest.
    • This was the oneriest task of the entire renovation project.
    • The oneriest part of the job was dealing with the endless paperwork.