Diabolos, in my experience, works best. Somnolence is the best pact by far for skilling up with bloodpact: ward. It's a magical attack, so it can't miss (bar things like magic shield of course) and has great chances of skill up. And you're right, rages are better for skilling (except somnolence, which is the same chance of course even though a ward). Camisado is pretty accurate, or you can always use nether blast which also cannot miss like somnolence. With good gear, diabolos is also the cheapest avatar to sustain because of his favor (ie gives you more refresh than carbuncle while out), which is also quite nice. So trade off Somnolence and camisado/nether blast.
One other nice thing is that if diabolos dies, somnolence gives a gravity effect. It makes it much easier to run away and summon.
So... stick with diabolos if you can for skill ups. He will get you them twice as fast.
Edited, Sep 22nd 2010 6:39pm by Annalise