Lanfear's been pretty much dismissed since Robert Jordan revealed that the 'finns had no power outside their dimension, but even before that Graendal was always the overwhelming favourite. Other mentioned possibilities with some credibility were Slayer, Sammael and Lanfear.

I've never heard of Ishamael being the perpetrator outside of theories on the Theory Post that predated my coming to this site by a fair bit which claimed that 'death took Asmodean' meant that 'Moridin took him'.