Go with Divine Robes and a weapon with Mana Up (Crystal Staff is good and relativly cheap). Id stay with that combination for all sized parties if your just gonna buff.
If your going with a smaller party (2-3 people including you) Id get a good pair of duals, +Str -Con tatoos and add to the damage. Divine Tunics will provide enough mana to buff/heal on occasion. There is nothing better than a duo with a prophet set up for battle. But thats my opinion. Some say Karmian with acu weapon... but I say if I can do 1/2 to 3/4 of the damage the rest of the fighters do, it makes a big difference in a duo.
EDIT: Prophets can solo in Heavy armor (devotion set for buffing themselves) and they only need to heal themselves every 2-3 mobs if you play it right.
Having karmian and an acumen weapon makes buffing a party so much more fun, atleast if you add +5WIT on that. Of course, you'll have far less mana and this setup might not be useful until you hit B grade when you have more base mana (but buffs costs more)... however, doing this you'll get skilled in managing your mana as efficiently as possible, and perhaps even learn to stagger your buff cycles into two parts ^^
Assisting in PvP as a buffer, having a larger mana pool with mana up weapon seems more beneficial, of if you want to play it lazy in PvE parties and don't want to buff each member uniquely to save mana... if you want to go with smaller parties or duos, a robe will still suffice but you should go for a melee weapon, something that uses your crit buffs - like a sword with focus and a shield, or dual swords... heavy armor for soloing as mentioned.
You can get the two handed sword as well, both work good its just a personal choice. The two handed sword looks cooler though.
edit: be prepared for dissapointment though. Karmian and Plated leather use the exact same model, it sucks big time. So you are trying to be different yet everyone assumes you are just like everyone else.
Well it all depends if the OP wants to buff large groups or small groups.
Big groups -> robes ; weapon -> mana up if you lack mana
Medium groups -> robes + decent melee weapon
Small groups/duo -> C Grade: Plated leather as above mentioned with berzerker sword w/focus, B Grade: BW heavy with +4 tsu*sls. Carry devotion set (NG) for buffing (+15% cast speed)
It all depends what you like to do. Frankly I dont enjoy buffing every 20 mins and heal people with crappy heals.
So Ive opted for: "I will buff you every 20 mins, I will make sure I have enough mana to buff everyone, I will hit the mobs the rest of the time and do almost as much damage as the rest of you. I will not heal, get a real healer for that" approach.