To be honest with you, I don't consider much of anything mentioned here as being "cult" in any true sense. To be a cult classic something has to have a small, but incredibly dedicated, following. Most of what's mentioned here is extremely mainstream, it's just that like most mainstream stuff it comes and goes, with a few fans hanging on. The stuff being listed as mainstream are pretty much blockbusters, utter phenomenas that represent an exceptional and lasting success. For the most part, if something has seen an official, licensed US release, odds are it's not "cult" in any way, shape, or form. After all that product had to be successful or promoted enough to get the attention needed to warrant the time, effort, and resources to see a US/English version created, and since that can be fairly expensive it says a lot about the product and the exposure it was getting from a business perspective.
Real "cult" Anime is going to be stuff generally created for late night slots and such that was considered to be a critical, or at least commercial failure in Japan, and was never even considered for a US release. The kind of thing you might find being fansubbed and more or less only available in any form through those kinds of channels. I'm not going to start shooting off titles or anything, since it would cause a lot of debate, and probably cause some people to go "I've never even heard of that" which would be kind of the point.
As odd as this will sound the "cult" animes most people are going to be familiar with are probably hentai titles. Something like "Urotsukidoji" was pretty famous especially when it was new, but was also kind of maligned and hated,
yet has surviving interest and a following even today.
Things like "Puni Puni Poemy" are odd animals, that was pretty much a stand alone sequel to "Excel Saga" which was a pretty big success that saw a US release, and then I believe a re-release as a "classic" given how well it went over. How relatively quiet it is, is because it was apparently involved in some porn seizures (Canada, and New Zealand were involved at different times if I remember) and arrests connected to the issue of whether a drawn minor involved in sexual situations can be considered actual child pornography. It's lack of western penetration seems to largely be
to concerns over distribution, so when it's appeared it's been relatively under the radar.
