A Bullet Journal
Bullet journals are great for having all of your important pieces of information in one place. You can customize your bullet journal, but an organized and detailed one will generally have these things:
Building a customized calendar and/or planner that works for you and helps to keep track of important appointments, your work schedule, vacations, major changes, etc.
Keeping track of your passwords and usernames (so that you don't scribble them down on random pieces of torn up paper - not only is this unsafe, but it also is very disorganized)
Making charts to keep track of things (some examples include budget trackers, a wish list, a grocery list, meal plan)
Daily and weekly to-do lists
Any super important things related to planning that you wish to include
A Moleskine Journal
For anyone who loves writing, a Moleskine notebook is wonderful for keeping track of things that you want to write about, ideas for future writings, and anything that inspires you in the moment that you don't want to forget about later.
A Pretty Journal With a Cover Design That Is Visually Appealing
For me, personally, DesignWorks Ink makes some pretty amazing journals with high-quality paper and gorgeous covers. I highly recommend them.
This journal is meant for you to express yourself honestly, pour out your heart, and write down unfiltered thoughts.
A Composition Book
This is recommended for keeping track of what you learn (such as coding, a foreign language, marketing, or any concrete skill).
A Small Notepad
If you don't carry all of your notebooks or journals around on a daily basis, a small notepad (you can find $0.50 ones at Wal-Mart) is very useful for jotting down quick notes that you can go over later. Or maybe, when you're on the phone and you need to write something down before you forget it, it's great to have a small notepad with you at all times.