So in this game, attacking is never really free, in order for a balloon to secure a hit it has to go through three different layers of defense:

1. It needs a way to get around a blocker or crush a blocker without trading.
2. It needs to not get avoided or removed by a battle action.
3. It needs to not hit and avoid or counter stamina effect.

So for a balloon to be a threat, it needs to have a real shot at securing a hit against a player, either because it's able to ignore some of these defenses or because these defenses are absent to begin with.

Generally speaking, any balloon is a threat when you have no blockers, no battle actions/breath to pay for them, and your stamina line is unlikely to have any avoid or counter effects.

At the same time, a balloon that is undodgeable is a threat without any blockers because it can't be answered by Dodge cards or the avoid effect.

In a sense, with the way this game works, strength is less about raw power and more to do with how effectively it can navigate enemy defenses

In theory, the best balloon is one that is both undodgeable and unblockable and can't be bounced or deflated. When that balloon attacks it's sure to get a hit. Still, it would be vulnerable to getting removed after attacking, and you can't count on getting your five hits off that one without ways to protect it.

Though another way a balloon can be a threat is if it threatens to remove defensive options. Such as getting rid of a blocker, discarding action cards from someone's hand, spending unused breath, or being able to check and shift out cards in the opponent's stamina line to get around avoid triggers. though some of these functions are likely better served on action cards rather than on-play effects for balloons.