I'm trying to optimize the damage done by a ranged Beast Master ranger, and was thinking of choosing the Stirge as a companion.
Blood Drain. Melee Weapon Attack: +5 to hit, 5 (1d4 + 3) piercing damage, and the stirge attaches to the target.
The Stirge will have 12HP (4x my level), 16 AC, +7 attack modifier, and do 7 points of damage per attack (all increasing with level and proficiency). It is not the creature that does most damage per attack, no, but consider the following part of the attack.
While attached, the stirge doesn't attack. Instead, at the start of each of the stirge's turns, the target loses 5 (1d4 + 3) hit points due to blood loss.
Can I command my companion to attack, and on the following turn leave it attached while I attack? This means another 7 auto-hit damage with it, my possible attack, and the stirge doing the Dodge action (per Errata '18 at "Ranger: Ranger's Companion"). The stirge will detach itself after draining 10HP, so I could pull this off every two-turns.
Up to 11th level, when companions can finally take multi-attacks, the Stirge seems a pretty solid companion. After that, probably better companions will exist.