GoodNotes - GoodNotes lets you take notes and annotate PDF documents.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Notepad by Braden Farmer - Notepad is an app by Braden Farmer that offers features that let you write and save plain as well as rich text notes by using HTML or Markdown.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Epsilon Notes - Common Mark editor for Android and Windows
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.