If you're interested to know what battledome challengers drop items of value, you can take a look at this list.
I've compiled in order of difficulty, and written down the prices as of 28th May 2020. Do let me know if you know of more
(you need an account to see links)
Harry the Mutant Moehog on Hard (56)
Drops: Moehog Transmogrification Potion (~90,000 nps)
(you need an account to see links)
Quiggle Warlord on Easy (69)
Drops: Gilded Winged Helm (~13,000 nps)
(you need an account to see links)
Lava Ghoul on Hard (92)
Drops: Lava Monster Stamp (~25,000 nps)
(you need an account to see links)
Jelly Chia on Hard (175)
Drops: Jelly World Stamp (~20,000 nps)
(you need an account to see links)
Meerca Henchmen on Medium (225)/Hard (300)
Drops: Shield of Coltzan (~20,000 nps) on Medium
Drops: Roast Chestnut Neggnog (~14,000 nps) on Hard
(you need an account to see links)
Lab Ray Scientist on Hard (518)
Drops: Experimental Physics (~97,500 nps)
(you need an account to see links)
Plumbeard on Medium (520)/Hard (780)
Drops: Golden Cobrall Spear (~17,000 nps) on Medium
Drops: Golden Compass (~19,000 nps) on Hard
(you need an account to see links)
Tekkitu the Witch Doctor on Hard (600)
Drops: Flaming Blooble Potion (~26,200 nps)
(you need an account to see links)
Snowager on Easy (650)
Drops: Frozen Negg (~90,000 nps)
(you need an account to see links)
Shadow Usul on Hard (700)
Drops: Shadow Usul Stories (~34,000 nps)
(you need an account to see links)
Mootix Warrior on Hard (844)
Drops: Cherry Mootix Lollipop (~180,000 nps)
(you need an account to see links)
Snow Beast on Hard (1200)
Drops: Cooking with Peas (~14,000 nps)
I only put items that cost above 12,000 nps but there were many other prizes in the 5,000 ~ 10,000s range.
BONUS
If you're willing to invest in Premium, then these 2 are the most worthy opponents
(you need an account to see links)(you need an account to see links)
Jetsam Ace on all levels
Giant Space Fungus on all levels
Drops: Nerkmid - all types (~73,000 nps)
General drop for the Cosmic Dome
(you need an account to see links)
Giant Space Fungus on Hard (636)
Drops: Bubbling Fungus (~35,000 nps)