Lead balloon is a well-known idiom used to describe a failed or unsuccessful project or event. While there might not be direct antonyms for this phrase, a few words can indicate the opposite of its meaning. When something is successful or achieves its goal, it can be described as a success, a hit, or a triumph. The opposite of failure or disappointment is success, which can imply a pleasant experience or a desired outcome. So instead of saying that a project was a lead balloon, one could instead describe it as a booming success or a resounding triumph, indicating a positive outcome.