My clan has the Skilling Bonus, Skillplot Buff, and Heal Over Time on our Avatars. These are the only ones that are useful for the most part, but the Skilling Bonus outweighs them all by far.
Unfortunately, most of the other buffs don't have very practical uses.
Also, Avatars are being reworked, so you may want to read through this thread about the proposed changes:
Clan Avatar: Feedback Wanted
~ Sicariu
Leader of