a list can be found here :
http://www.maladjustite.com/dom2/SCQRv10.pdf
just additional info trying to be complete ( most of it you will know probably already
):
many people say that the vastness is the best summoning . but you only get a few . in a game i played on orania as ryleh i got 2 until turn 90 or so.
if you are lucky and your void summoner survives long enough after a big enough summoning skill you mostly get greater othernesses which are the second best void summoning and perhaps if very lucky another vastness .
the vastness has 10 strategic movement so gift of reason her and make her a sc which can travel almost everywhere without needing to use a spell like teleport .
don't use mages they get almost always feeble minded . priests instead only very rarely . on a post i read mages 75% per successful summoning and priests only 3% .
if you are very lucky you can cast the gift of health (enchantment5,ritual) and keep it and then when your void summoner becomes feeble minded normally after 1-2 turns he is healed again .
normally you get one successful summoning every 3 or 4 turns . give your summoner 5 bodyguards or you lose him sooner or later on a random void monster attack which is similiar to an assasination