The antonyms for the phrase "most jam packed" would be "least crowded", "spacious", "uncluttered", "empty", "unpacked", or "unfilled". These words present the opposite meaning of a tightly packed or overfilled space. For example, a spacious room with plenty of breathing room would be the opposite of a jam-packed room. An empty table would be the opposite of a table that's filled with books, papers, or other materials. The use of antonyms can help add emphasis or contrast to writing and provide a variety of ways to describe a situation or setting.