Ahoy maties!
I Spearedyou here once again with a great report on the levels on a lot of the monsters and creatures in the free part of Runescape. This can help you to determine which monster would be the best for you to train on.

Goblins Level 2, 5, 11, 13, 16, and 25
Man Level 2
Woman Level 2
Barbarian Level 9, 10, 15, 17
Gerturd the Great or something i forget his name Level 26
Rat Level 1
Giant Rat Level 3, 6, 26
Spider Level 1
Giant Spider Level 2, 27, 50
Lesser Demon Level 82
Greater Demon Level 92
King Black Dragon Level 270-something
Green Dragon Level 79
Red Dragon I forget
Cow Level 2
Cow Calf Level 2
Wizard Level 9
Dark Wizard Level 7, 20
Guard Level 19, 21, 22
Dwarf Level 10, 11
Barbarian Woman Level 8 or 9 i forget
Scorpion Level 14, 50-something
Catablepeon Level 64 and I think 67
Skeleton Level 21, 22, 25, 45, 68
Flesh Crawler Level 28, 35
Zombie Level 13, 14, 30, 44, 54
Minotaur Level 12, 27
Wolf Level 11
Jail Guard Level 26
AL Kharid Warrior Level 9
Imp Level 2
Tree Spirit Depends On Your Combat Level
Rock Golem Depends On Your Combat Level
River Troll Depends On Your Combat Level
Evil Chicken Depends On Your Combat Level
Special Zombie Depends On Your Combat Level
Tramp Level 2 I think
Dark Warrior Level 8
Black Knight Level 33
White Knight Level 36, 37, 38
Duck Level 1
King Scorpion Level 32 I think
Hill Giant Level 28
Moss Giant Level 40-something I think
Snake Level 5 I think

That's all i have so far if you have any questions, message me on Runescape u know my username.