What do you think are some of the best forms for an Adult (or older) Bronze Dragon to take?
Combat-wise?
RP-wise?
Spy-wise?

Love to know your thoughts.
~Mad

Lawful good. Temperate climates. Hills are its favoured terrain. They enjoy interacting with humans like at festivals and what have you.

Kinda just skimmed over countries with temperate climates and figured they would be very handsome/beautiful individuals with a joy for swapping tales in human form. If they are unsure of people trespassing within their protected domain, they might take the form of an animal that is native to the region and keep the group under wraps and only approaching when he is sure of their nature. Should he need a more in-depth talk with the party in question, he might take the form of a handsome figure native to the region or copy a form he once saw and pass himself off as a wandering tale-teller where he could trade a story in exchange for theirs to see what kind of people he is interacting with? From there he can try and get a better insight at how they react and speak to him.

If the region was quite bumpy with rolling hills and uneven, I could imagine the dragon taking the form of a very strong mountain goat to deter people from hunting it while blending in with the rest of the tribe. Or possibly even just a sheep form blending in with the flock, with cliffs overlooking the sea close to where its lair presides; potentially right underneath a shepherd's home and the owner doesn't even realize it.

Combat-wise, I am clueless on that prospect.

Bronze Dragon spend much of their time in the ocean or along coastlines, so aquatic animals would most likely be their preferred shape change options. They are known to saving drowning people in dolphin form and probably use that form for underwater exploration, along with other creatures that use echolocation like killer whales and speed whales. A sea gull would be good for scouting out both ships on the sea and coastal/port towns. Small animals typically found in humanoid cities (rats, cats, dogs, spiders, etc.) would also be fine choices for urban exploration. 

For offensive forms, you have things like the sperm whale, the giant lightning eel, obliteros (huge shark from Water Deep Heist), hulking crab, killer whale and the giant walrus. 

For role play? Well remember that a dragon using shape change can still speak, keeps its monstrous hit point, has all it’s lair actions and legendary resistance, and uses it’s own mental stats while in a beast form. So you have a lot of creative liberty as to how you can use a dragon’s shape change for roleplay. 

Combat:

Itself

Roleplay:

humanoid, perhaps a charismatic fisherman, considering bronze dragons live primarily off of fish.

Spy:

Beach mouse because beach mouse

***Minor Spoilers for Storm King's Thunder***

Combat-wise I'd probably go with huge @!%&ing dragon...