The power armour suits worn by Chaplains are considered holy in themselves. Often they are hundreds, if not thousands, of years old.
A Chaplain's armour is highly stylised and archaic in appearance. Black is the colour associated with Chaplains, and most or all of their armour is painted in this colour. Symbols such as skulls and Imperial eagles commonly adorn the armour, but Chapter-specific symbols are also used. The face plate or the entire helmet is traditionally skull-shaped. Skulls are also repeated throughout a Chaplain's armour - adorning an entire shoulder plate, the upper chest armour or the groin guard. One area of the armour, such as a shoulder guard, is always left in the Chapter colours, displaying the Chapter symbol.
Part of the Chaplain's formal regalia is the crozius arcanum, the Chaplain's staff of office. Used in chapter ceremonies and worship, many Chaplains take them into battle, symbolizing that battle is the highest ritual of the Chapter. The crozius is usually topped with an Imperial Eagle or winged skull. It incorporates a potent energy field, allowing it to be used as a weapon; one that will punch through armour easily, showing the power of faith.
The Rosarius that Chaplains wear around their neck incorporates a powerful force field generator, offering the bearer protection against weapons that even power armour would offer no hope of survival against. In light of this, the rosarius is sometimes referred to as a Chaplain's 'soul armour'.
Master of Sanctity Faust
I feel as though the Chaplain is the best of the three choices you have to make when it comes to a Space Marine Chapter's HQ choice.
Yes you can have a Captain or even a Chapter Master for about the same amount of points...throw them in some terminator armour give them a power weapon and a storm bolter...but they won't have that 4+ invulnerable save and it cost an extra 10 points. On top of that you have the Liturgies of Battle as well as the Honour of the Chapter...The Liturgies are a very useful item to have when assaulting which is something I try to do whenever I get a chance...and seeing that this guy is in Terminator Armour from the word go on top of atleast a 5 man terminator squad I am sure they will be packing a nice sized punch for anything or anyone they come across.