A common battle tactic among mid-level spellcasters is to remove one big enemy from the fight using polymorph to transform them into a harmless beast. However, the question inevitably arises: what beast should one choose? The obvious answer that most people come up with on their first try is something tiny, like a rat or an insect. This certainly takes away any ability the creature has to deal any damage, but the mistake is quickly revealed when a single errant point of damage causes the enemy to revert to their original form.
So, the ideal choice of form for "offensive" use of polymorph on an enemy has two properties. The first obvious one that everyone intuitively grasps is that the form should have little or no ability to attack or otherwise influence the battle. The second one, which people tend to forget until their first big mistake, is that it should be durable enough that reverting the form via damage is difficult.
Which beast offers the best combination of harmlessness and durability?
Sources to compare include the Monster Manual and the Players Handbook.
Note that high HP and weak attacks are not the only way to satisfy these requirements. For example, "durability" could be satisfied by high AC and other defenses. Likewise, "harmlessness" might be achieved by choosing a form too slow to catch up with anyone, even if it has a powerful attack.