All righty, this is coming from someone who's gotten four (almost five

) quest capes and done thousands of quests:
-Quest (Benefits)
-Lost City and the gnome series (Dragon dagger/longsword/scimitar, gnome gliders, spirit trees)
-Fairytale Pt I, at least start a few steps of Pt II (Fairy ring access)
-Elf quest line, at least up through Roving Elves (Dragon halberd, massive unlocked map area - especially useful for clues. Crystal equipment isn't that good, but you do get it, too)
-The Fremennik Trials/Isles (Access to useful helms and various quest prereqs)
-Recipe for Disaster (Barrows gloves, varied XP, helpful bank/item shop in the Lumbridge Castle basement)
-Animal Magnetism (Ava's attractor/accumulator)
-The Creature of Fenkenstrain (Ring of Charos (a))
-In Search of the Myreque (shortcut to Mort'ton/Barrows, prereqs offer access to Morytania)
-Desert Treasure (Ancient Magicks)
-Dwarf Cannon (multicannon use)
-Tears of Guthix (free weekly XP in your lowest skill)
-My Arm's Big Adventure (extra herb farming patch, and Eadgar's Ruse (a prereq) will get you Trollheim Teleport)
-Legends' Quest (Kharazi jungle access, ability to wear Dragon sq shield)
-Lunar Diplomacy/Dream Mentor (Access to Lunar spells and Lunar isle, currently the fastest Runecrafting XP in the game)
-One Small Favour (Steel key ring, two useful XP lamps)
-The Slug Menace (Proselyte armor, some of the best Prayer bonus in the game)
-Cabin Fever (access to Mos'Le Harmless)
-Shilo Village (access to Shilo Village :3)
-Throne of Miscellania/Royal Trouble (manage your kingdom for decent daily profits)
It's a pretty exhaustive list, I think

Have fun..