If your player character has access to the Polymorph spell, there are a few viable options from which to select. Of course, it all depends on the situation. I’ve broken down scenarios into three categories and will share my experience with each. And since I’m a great windbag, this will be a three parter. So stay tuned for the rest!
Scouting
Underground: As most campaigns take place in a dungeon or similar environment, I usually opt for a Bat if I’m going for stealthing and/or scouting. There are a couple reasons for this. Number one, I’m a bit of a purist. Bats probably wouldn’t draw too much attention from dungeon dwelling creatures, as they are most likely usual residents. Furthermore, they have blindsight up to 60 feet and a passive perception of 11. Their Keen Hearing feature gives them advantage on Perception checks that rely on hearing, seeing as how they use echolocation to locate objects and what not. You can find more on the bat option on page 318 of the Monster Manual.
Above Ground: A similar screener went into choosing the best option for scouting above ground as well. Thus, I’ve chosen the Hawk. With a passive perception of 14 and a flying speed of 60 ft, it can cover a lot of ground rather inconspicuously. Similar to the bat, its Keen Sight gives it advantage on perception checks that rely on sight, as opposed to hearing. This can really come in handy when patrolling for enemy encampments and the like.