10. Kamil (Desert Treasure) - Level 154
By a hair, over Dessous, Fareed, and Damis.
9. Elvarg (Dragon Slayer) - 83
The one. The only.
8. Nezkichened (Legend's Quest) - Level 187
Difficult at the time, not so much now.
7. Decaying Avatar (Nomad's Requiem) - Level 525
The other boss from Nomad's Requiem has 25,000 LP and higher defence than Nomad, and can hit well into the 400's. However, his attacks are made much less deadly by the fact that there are safespots and that he cannot hit through Protect from Melee. However, his defence is so high that the boss fight can end when the player runs out of supplies, and one misclick can result in healing of up to 10,000 lifepoints. This battle tests the concentration of the player. If you, as I did, start paying attention to the chat, you're in danger of having to restart the entire fight over again.
6. Zenevivia (Love Story) - Level 246
Don't let the looks fool you - Zenevivia can easily kill an unprepared adventurer (such as myself on the day that this quest came out). She has a deadly fire pit spell that can deal a lot of damage as well as Blood Barrage. Also, she's not easily meleed without a Dragon or Zamorakian spear to knock her off of her pedestal. If tips and tricks are known, Zenevivia is fairly easy, however.
The top 5 are clearly more powerful than anything before them. These are the only boss fights that might be challenges for a higher-level player.
5. The Inadequacy (Dream Mentor) - Level 343
I personally found The Inadequacy to be extremely easy. However, my friends have assured me that's not always the case. You can start off with its high-200s max hit that's accurate and can't be prayed upon (get the pun?). It also summons level 78's called "A Doubt(s?)", which can hit in the mid-100s.
4. Balance Elemental (While Guthix Sleeps) - Level 454
The Balance Elemental is essentially the hardest "standard" quest boss. It has quite a bit of LP (2750) and hits 300+ with all styles. There are no gimmicks. Like many of the boss fights, this one is DDS fodder if you're on a high level, or a struggle if you're just around level 100.
(If I had chosen to list all bosses from one quest in one article, Dream Mentor's bosses would have gone here.
3. Dagannoth Mother (Blood Runs Deep) - Level 353
This is a pretty annoying boss for two reasons. First of all, the new autocast system makes it difficult to cycle between spells. Second of all, it hits like a truck if you fumble with the interface. Which you will. Oh, and also, you have to fight two level 192 Dagannoth Sentinels beforehand unless you want to bank over and over again.
2. Pest Queen (The Void Stares Back) - Level 599
The Pest Queen, the newest Quest Boss in RuneScape, is also probably the second-most powerful. It absolutely murders anyone who doesn't have a clue about what to do or doesn't pay attention to the whole screen. Also, without a high defence level, it can hit very hard. In addition, it takes the player by surprise if they haven't been prepping for the fight by reading a guide. All this adds up to about 1/4th of the best monster on the list.
1. Nomad (Nomad's Requiem) - Level 699
Surely, you didn't expect to see anyone else at the top of this list? Nomad's 4 attacks in main form (magic, landmines, soul blast, and INFINITY MINUS ONE BLAST) are all capable of hitting 300+, lag results in death, and a simple misclick to one step can also end catastrophically. Oh yeah, and it has 12,500 life points. To make matters better, items are regained upon death, but this is still by far the most challenging Quest Boss and one of the most challenging in the game.