The Medium Varrock Diary has one of RuneScape gold the most convenient QoL upgrades in the entire game.
Key Reward:

Ability to toggle the Varrock teleport to land directly at the Grand Exchange

This seemingly small change greatly increases banking efficiency and is invaluable for both Ironman and main accounts,especially when crafting,skilling,or flipping items.
Kourend & Kebos Achievement Diary

The Elite Kourend & Kebos Diary is among the most challenging in terms of requirements,but the rewards are well worth the grind,particularly for Slayer and bossing.
Key Rewards:

Unlimited teleports to Mount Karuulm

No longer required to wear Boots of Stone/Brimstone in Mount Karuulm

Bonus Slayer points from Konar tasks

Access to Alchemical Hydra with upgraded boots like Primordial Boots

Rada's Blessing 4: Best-in-slot ammo slot item with versatile utility

Rada's Blessing 4 offers:

+2 Prayer bonus

Slight chance to catch a double fish

 Increased drop rate from tasks and monsters

Extra Prayer XP with Ash Sanctifier (when combined with the hard diary reward)

You'll also benefit from a 10% reduced chance to burn food in the Hosidius Kitchen,making it one of the most efficient cooking locations in the game.
Lumbridge & Draynor Achievement Diary

The Elite Lumbridge & Draynor Diary is one of buy OSRS gold the best diaries in terms of account utility,although its high stat and quest requirements make it a later-game goal.