Here are some of the most popular rewards/creatures to kill
Level 1:
Goblins and men are the best for these
You can get black (t) and black (g) from these level clues, as well as different colored berets.
Level 2:
Guards and jogres, paladians work too but they have a good attack so you'll be taking a lot of damage.
You can get adamant (t) and adamant (g) from these, also the almighty
Ranger Boots
Level 3:
Blue dragons, greater demons, hellhounds
You can get rune (t) and rune (g) from these. Also, Gilded Armor (solid gold, extremely rare and extremely expensive
), Robin Hood hat
, and cavaliers
All level clues can give you torn pages (used for prayer books)