Question: I need a digital note-taking system that allows me to create hierarchical notes and filter them easily.

Supernotes is a collaborative notes app that can handle rich formatting like Markdown, LaTeX, images, emojis and tables. It's got a lightweight hierarchy for organizing your notes, too, and powerful filtering abilities. Supernotes is built for productivity, with AI-powered typo spotting and idea grouping, so it's good for note takers, researchers, writers, students and pros. It's available in free and paid versions, with offline access and multi-device support.

Another contender is NoteGPT, an AI-powered knowledge management tool that can help you learn more efficiently with AI summarization, auto snap for note-taking and AI-generated flashcards. It's also got powerful note management abilities, like filters, search, folders and tags. NoteGPT is good for students and pros who need to digest lots of information and keep a digital library of information organized.

If you want a tool that's more of a combination of notes and task management, IXORD could be what you're looking for. It can handle text and voice messages, advanced block-style formatting and a tagging system to organize your notes. IXORD also has AI-powered task explanations, a collaborative agenda and a document hierarchy, so it's good for freelancers, businesspeople, students and creative types. It's got a free trial and a basic subscription at $7.99 per month.