The Fremennik Trials
Reward: 2,813 Defence Exp, Ability to wear Fremennik helmets (Berserker, Archer, Farseer, etc…)
(*** CAREFUL***, I chose not to do this quest, because if you hybrid PK, you're better off with no helm, a Mystic/Enchanted hat, or full Void, with interchangeable Helmets. GL.***)

Dragon Slayer
Reward: 18,650 Defence Exp, Ability to wear Rune Platebody and green dragon-hide body

Monkey Madness
Reward: 20,000 Defence Exp (***Once you've finished the quest, do not talk with the trainer to receive XP in the training camps. After you're done ALL the required Berserker Pure Quests, and you're not yet 45 defence, calculate how much you'll need, then go back for your experience.) Ability to wield the Dragon Scimitar, Require for part of Recipe for Disaster.

Nature Spirit
Reward: 2,000 Defence Exp, a required sub-quest for Recipe for Disaster

Holy Grail
Reward: 15,300 Defence Exp

In Search of the Myreque
Reward: 600 Defence Exp, Shortcut to Barrows (a decent money-maker)

Heroes Quest
Reward: 3,075 Defence Exp, Ability to access Heroes Guild, Ability to wield Dragon Battle Axe and Dragon Mace

The following list of quests aren't necessary, but are useful. Some will save you money on Prayer, some will give you special abilities, and some will unlock new tiers of armour/weapons which may be useful to you in quests, slayer, or PK-ing. GL.

Holy Grail
Reward: 11,000 Prayer Exp

Swans Song
Reward: 10,000 Prayer Exp, Ability to fish Monkfish

Making History
Reward: 1,000 Prayer Exp

Mountain Daughter
Reward: 2,000 Prayer Exp, Bear Mask

Recruitment drive
Reward: 1,000 Prayer Exp, Ability to wear Initiate Armor

Restless Ghost
Reward: 1,125 Prayer Exp

Priest in Peril
Reward: 1,400 Prayer Exp, Ability to access the Kingdom of Mortania

Ghosts Ahoy
Reward: 2,400 Prayer Exp, Ectophial (1-click teleport), Ability to access Port Phasmytus free of charge
Rag and bone man
Reward: 5,500 Prayer Exp

Rum Deal
Reward: 7,000 Prayer Exp

Spirits Of The Elid
Reward: 8,000 Prayer Exp

The Great Brain Robbery
Reward: 6.000 Prayer Exp, Ability to wield the Barrelchest Anchor (The Anchor is fun to mess around with. However, the Prayer Experience is what you're after.)

My own opinion on Prayer XP: Buy your own Dragon Bones, and use a gilded altar with 2 incense burners. Faster, although more costly. Go for either 43 prayer, 44 prayer, 45 prayer, or 52 prayer. They are all good for different reasons. 95 is for Turmoil :).