SB800 (discontinued by Nikon) is a good choice if you want to really learn how to use a flash, including bounce flash, backlight flash, multiple flash etc. Buying two SB800's is recommended, both can communicate with each other and the camera for TTL (Through The Lens) exposure on various lighting setups.
SB400 (discontinued by Nikon) is the best option if you just want a flash that works like a camera built in flash. It's small, light and takes 2 AA batteries (recommend rechargeable enerloops), decently powerful for it's size. Other small Nikon flashes often use AAA batteries with less power and fewer flashes.
SB-5000, Godox AD200 etc are for when you have already learned what flash can do to improve your indoor studio and outdoor macro pictures; I doubt you would be asking the question if these would be the best for you.
SB-800 and SB-400 can be purchased used off ebay, or B&H Photo etc. There is an
SB-400 and
one,
two SB-800's available now in the used section at B&H.
Edit to add: Try to get a manual if you pick the SB-800 flash as it can be problematic trying to correctly configure the flash for some lighting setups.