Which beast should I choose to most effectively remove an enemy from a fight using the Polymorph spell?

Question

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.

Answer 1

Pick an underwater creature.

There is an undeniable correlation between health and damage, making it impossible to pick a tank that deals no damage.

However, you can pick something, such as a killer whale, that has no movement on land, so it cannot deal that damage. Some underwater creatures will suffocate after a number of minutes equal to their con modifier (min 30 secs), but that is enough time for combat to pass. This is not the case for a killer whale, however, as it can breath above water, being a whale (thanks Chris Starnes)

This makes a hefty, but useless, beast that can't do anything, unless you get in melee range (not recommended).

Answer 2

I'm not sure any Beast really fits your requirements.

A "single point of errant damage" isn't something that can really happen in the game; weapon attacks are targeted at a single enemy, so the most common way for this to happen is accidentally catching the creature in a spell's area of effect.

Low-damage area effects like burning hands are generally in the 3d6 range, so you're talking about around 11 damage on average. While you might roll a 3 or 4 for damage or the beast might save successfully, those are unlikely; I'd say anything with 10 or less HP is not reliably more safe than a simple rat. And at the other end of the range, with a fireball throwing down 8d6 (28 on average), nothing short of an elephant has much of a chance of survival.

The best mix of inoffensive but hearty might be something like a giant toad, but even then you're talking about something with a decent chance to eat somebody. Maybe a cow (Volo's Guide to Monsters) would work, but at 15 HP you're still well within the range where even one burning hands has a decent chance of taking it out.

Generally, Wizards of the Coast doesn't produce creature stat blocks that are a long way out of whack in terms of hit dice versus offensive power. Nobody wants to slog through a fight with some monster that can't do any serious harm but takes forever to kill (and conversely, weak creatures that deal lots of damage are often seen as unfair or worthless depending on whether they can get in the first shot). The perfect creature for your polymorph requirements is precisely the kind of creature WotC wouldn't print.