Choosing a ball for Muk? Compare the normal and shiny recommendations below with their reference sprites. “Best” here means a colour or theme match, not improved catch odds.
The community colour-match recommendations for Muk are Dream Ball, Master Ball, Dusk Ball.
The community colour-match recommendations for shiny Muk are Nest Ball, Friend Ball, Dusk Ball.
Choose the appearance you prefer from the listed options. Normal and shiny Muk can suit different balls; these are community suggestions, not a measured ranking.
Source: Legal Matching Pokéballs 2.1 by OracleLink. Community recommendations, not a per-game legality checker. Availability depends on the game, form and encounter.