Well, the best Ulver album to start with is going to depend on your personal tastes. Theres 3 eras of ulver really.
The first era, is the black metal/folk music era. Lots of black metal guitars, and blast beats, but lots of acoustic passages and atmospheric elements, all in a woodsy and natural feeling. this is my personal favorite era.
The second era is when Ulver lost the black metal and folkish roots, and started to experiment with different types of music and elements in their sound...pretty much evolving into an electronic/rock hybrid of sorts, but with plenty of experimentation. Also a good era for those with eclectic tastes.
The third era, is where Ulver has pretty much lost all their rock/metal elements alltogether, in exchange for a completely melodic/ambient/electronic or symphonic aproach. This era includes soundtracks for films, as well as their own original studio creations with various subject matter. This is my 2nd favorite era, for its maybe the most creative and accesible material for them.
so in conclusion, i took the liberty of recommending my favorite album from each era, and you can use my brief summaries above as a guide.
era 1: Bergtatt
era 2: Themes From William Blake's - The marriage of heaven and hell
era 3: Perdition City