Arsenic is a naturally occurring substance, found in many soils around the world. Takes too long though & is readily detectable. Nasty way to die, too.
Cyanide is mucho quicker but, once again, easily detectable.
I'd try anti-freeze, slowly increase the amount & pow, history is made. It can also be detected by microscopic liver tests but there would have to be suspicion involved.
Act natural, pretend you really miss her, make sure you don't have some giant insurance policy out!