Most tidal refers to the highest or strongest level of tides in a particular area. The term most tidal, therefore, has no direct antonym or opposite word. However, one can use words that describe the opposite of tidal, such as still, stagnant, or calm, to refer to areas where there is no or minimal tidal activity. These areas may include small lakes, ponds, or any body of water with little or no influx or outflow of water. While most tidal describes the highest level of tides, a word like least tidal would describe an area with minimal tidal activity. Therefore, the antonym for most tidal is least tidal.