Treatises, jurisprudence, law review, and law journals provide foundational perspectives on tax law that can inform and inspire researchers. So, who has the best offering?
An initial statement would be that if you have a specific treatise company that you like, such as Carolina Academic Press's Understanding series or Westlaw's Nutshells series, I recommend finding ways to access these through either library/interlibrary loan or through the subscription service (as you can do with Westlaw and Nutshells). The same thing goes for law journals, jurisprudence, etc.
With that said, LexisNexis, Westlaw, and Bloomberg all provide robust collections of secondary sources that are all pretty comparable in tax law.
LexisNexis includes separate links for their tax treatises, guides and jurisprudence and their tax law review and journal offerings. These are very well organized and, while not the most user friendly in terms of a listing instead of putting everything--albeit in alphabetical order--on a single page, provide search bars to quickly find what you are looking for.
Westlaw has a much more appealing design to their tax secondary sources. However, these sources are compiled, unlike LexisNexis's, and thus it is more difficult to determine whether something is a law review, law journal, or treatise just by merely looking at the title alone. However, it is usually easy enough to figure out. Thus, the sacrifice is clarity in source type for more clarity in user interface, which is more valuable, in my opinion.
Lastly, Bloomberg includes a books and treatises section on their tax law practice area start page. While it is helpful to have all tax documents and search functions having a genesis on a single page, the options for these secondary sources are extremely limited compared to Westlaw and LexisNexis and thus it is not superior in this field.