You have asked about the best way to parse HTML, so I think HTML agility pack is the best way.
No, it is not the default option for zennoposter...
But sure You can use parsing module what is zenno default option as well.
Not sure what You mean compatible with different sites as You scrape exact content from exact site usually.
But using Regex is fine for scrapping.
Take a look at Parse brick in zenno...
Right click on content You want to parse and ''Parse content''
I try to explain my need more clearly. Sorry English is not my native language. I need to scrape the content of some HTML tags and keep the article with the same structure as the original. The problem with the "parse data" module is that I will have all the h2 tags together all the p tags together... I want to have a result of type :
h2
p
h2
h3
p
h3
p
h2
If we talk about scrapping data then how to use regex is what You have to learn. Wiki - Regex
Parse content is just helper action. So go with Regex and learn it.
About logic for Your scrapping, I can not answer much, because I do not understand what is Your goals...