Most Maran powers are primarily for convenience. They help noobs who cant properly prep/gear themselves maintain a quasi-competitive presence by handing them the basics.

Mantle of the phoenix is mostly rendered moot by virtue of Flight/Prot evil potions.
Lightforge is my personal favorite since I usually hemorrhage high quality weapons.
Zeal is basically berserk/frenzy minus the drawbacks.
Phoenix brand is fairly useful in PvE, but is hard to get off reliably in PvP
Cry of deliverance is unreliable, and has many restrictions.

Contrasted with Aco powers which open up a few new doors, but tend to be very situational/unreliable.

Favor of the sun is useful, but it is virtually impossible to tell what resistances you have while prepping, which can lead to wasted preps/time.
Spirit Link is amazing (if you are a priest). If you are a bard/mage/warrior, its practically useless.
Spirit sight is useful for when you are blind/dont know the person sending you tells, and need a quick measure of their general trustworthiness.
Altruism is a useful tool for debuff/damage management in PvE and noncombat situations. Makes certain Aco builds so much more viable. Situationally useful in PvP.
Succor is pretty good, but it has very significant drawbacks.
Curse of radiance is amazing, and underused despite its situational usefulness. It doesnt do jack in PvE.
I have a love/hate relationship with Bless Water.