I would say Drakkin. I have not encountered many Drakkin necros over the years. Also male Erudite necros seem to be rare.
These days? Human, haven't seen a Human Necro in years. I'd say it goes something like this... Dark Elf (by far and what most think of when they hear Necro) Iksar (Look and old school Regen factor for Lich) Gnome/Erudite (All those people who want to be different) Drakkin/Froglok (Newer toons/Race changes for the look) Human (Meh in every way from stats to look)
Don't think I've seen a Guktan necro....ever. That combo exists? ^ . ^ Frogger robes looking meh probably plays a part!