The phrase "most heaving" can be replaced with various synonyms depending on the context. For instance, the word "crowded" can be used to describe a gathering of people or a public transport facility. "Packed" can be used to describe a place that lacks space to move freely, like a cinema during a blockbuster movie's premiere. A "dense" situation denotes a high number of people or objects in a confined space, while "thronged" connotes a large group of individuals occupying a specific place. "Teeming" can be used to describe a place overflowing with people or things, with "overflowing" emphasizing excessiveness or abundance. In summary, several alternative words can be used instead of "most heaving" to communicate the situation effectively.