List criteria
Here is a list of all the passive regeneration skills and traits that I can find on the wiki. The criteria for this list is that the health must be regenerated by simply having the skill or trait, without an active effect or condition. This is why traits such as the Theifs Shadow's Rejuvenation did not make this list.
Skills and Traits
Warrior, Healing Signet, Adrenal Health
Guardian, Virtue of Resolve
Ranger, Signet of the Wild
Engineer, None?
Thief, None?
Necromancer, None?
Elementalist, Soothing Mist
Mesmer, None?
So which is best?
Let's take a Healing Power of 100 and a time of 3 seconds.
Healing Signet,
At level 80: 200 + (Healing Power * 0.033) per second
= 609.9hp
Adrenal Health,
1 Bar of Adrenaline - 125 health every 3s
2 Bar of Adrenaline - 240 health every 3s
3 Bar of Adrenaline - 360 health every 3s
= Answers in the question ;)
Virtue of Resolve,
0.06 * Healing Power + 84 per second
= 270hp
Signet of the Wild,
62+6% of Healing Power, per second
= 204hp
Soothing Mist,
Level + 0.05 * Healing Power per second
= 255hp
How does this compare to Regeneration?
Regneration boon
130 + (0.125 * Healing Power) per second
= 427.5hp
Conclusion
The Warriors Healing Signet gives the best passive regeneration.