1.) Dust Devils
Slayer Level Needed: 65
Monster Level: 93
At Level 65 slayer, you will be able to kill Dust Devils located in the Desert Slayer Dungeon, in the Chaos Tunnels, and amidst the smoke to the far west of Pollnivneach. These are a very common monster for Slayer trainers and masters to kill because of their potential drop of the Dragon Chainbody, which is currently worth 4.5 million GP. They also have the potential of dropping the left half of the Dragon Sq Shield, teeth and loop halves of keys, many types of herbs, and many other smaller items.
2.) Spiritual Magers
Slayer Level Needed: 83
Monster Level: 120, 121, 122
Located at the Godwars Dungeon, Spiritual Magers range from level 120-122. There are three types of Spiritual Magers: the level 120 Saradomin Spritual Magers, level 121 Zamorak Spiritual Magers, and level 122 Armadyl Spiritual Magers. Spiritual Magers have the potential to drop Dragon Boots and Dragon Spears, along with many other smaller items. Although the drops are less valuable than what the Dust Devils can drop, they are less rare and you can potentially earn much more from them in the time it takes to get your hands on a Dragon Chainbody from a Dust Devil.
3.) Abbysal Demons
Slayer Level Needed: 85
Monster Level: 124
Abyssal Demons are located at the Canafis Slayer Tower and the Abyssal Space, which can only be entered by using Fairy Rings. At level 85 Slayer, you have the chance to get them as an assignment or just to kill them for drops. Their highest rewarding drop is the Abyssal whip, which is currently worth around 2 million GP. Abyssal Demons have very well-rewarding smaller items ranging from 20k to 80k in value, like the Dragon Med Helmet, Dragon Spear, all different kinds of Rune armor and weapons, and both the tooth and loop halves of keys, which make this monster an easy way to make money fast no matter which drops you get.
Basically, when you look at the Slayer skill, you can easily see that time equals money and experience. Everything you do with the skill will earn you money and experience in the skill. However, Slayer is a very slow-paced skill, meaning reaching 99 will take a very long time. To get to 99 without freaking out or without quitting the skill entirely because of how bored you may get, take the skill and train in blocks. Train for a certain amount of time and then take a break. Without doing this, you may start to get frustrated easily and end up losing money.
If you aren't going for the 99 Slayer Skill cape and you just want to earn money, take a look at the top 3 slayer monsters just mentioned. Pick one and make it your target goal to get to. Once you get high enough to kill that certain slayer monster, use it as your main source of training any type of skill and you will see the money roll in.
The last, and most important piece, of information to take from this guide is to keep every drop! Even if its 10 fire runes or 1,000 coins. Everything in the end adds up and it's the only way you'll to earn maximum GP from Slayer training.