To me personally, the most important script was Self-Study Quiz, combined with Self-Study Hide Info, but it depends on your study preferences.
You are still in early levels, so it’s going to be relatively easy for a while, but you’ll eventually find it more difficult to remember all of the info you are packing into your brain. I found it immensely helpful to start each new level – while items are still at Apprentice rank – by repeatedly recalling items until I could recite the reading and meaning without hesitation. It takes more time up front, but you’ll find that your Reviews go much faster (at least 2x for me), and you will forget less. In Self-Study Quiz, you can create a quiz preset that includes your current level’s items (put “+0” in the Level field of the Items Preset). Setting up the quiz for Reading-Then-Meaning also helped me immensely. About half of your brain’s neurons are related to sequence-learning, so quizzing the items with “Reading-then-Meaning”, back-to-back, helps cement the two together in your mind. Some people find “Meaning-then-Reading” more effective, but I prefer ‘reading’ first because it makes me think of the Japanese pronunciation first as soon as I see a word, which means you are closer to ‘thinking’ in Japanese.
Self-Study Hide Info is designed to let you use the Items pages on Wanikani (e.g. the Levels, Radicals, Kanji, and Vocab pages) as a simple quiz without the Reviews-like interface of Self-Study Quiz. Basically, it hides the Reading and Meaning on those pages, and you can go item-by-item trying to recall them. Then hit the Shuffle button and repeat until you can recite them without hesitation. Shuffle is important because otherwise you’ll start remembering things by the sequence that they appear on-screen, which gives you artificial help that isn’t helpful for real-world memorization.
Both of those scripts are slightly broken: Self-Study Quiz relies on certain font-based icons that WaniKani removed from the site a few years ago, so most of the icons won’t appear in the Quiz interface unless you modify the script. Some helpful users have posted info in the Self-Study Quiz thread on how to patch the script. (I don’t have time to do an official fix.)
Self-Study Hide Info is also slightly broken: WaniKani used to fully load each new page as you navigate, but they changed the site to where it loads pieces of the site dynamically as you navigate around, so the Hide Info script doesn’t get started when you navigate to one of the pages where it is supposed to run. So, you have to navigate to one of those pages, then click the browser’s refresh button, and the interface will appear.
In the past, there also used to be a script that would let you quiz your items in the official WK Reviews with the Reading and Meaning given back-to-back. But, again, WK changed how the quiz queue works, so I’m not sure that’s possible anymore (at least, not without someone writing a new script to do so). That script also made a huge improvement in getting Reviews done quickly. Some people said they found that it negatively affected their memorization, but my sense from people’s feedback is that it didn’t harm most people’s memorization, and almost universally made their reviews go faster. But it’s no longer possible now anyway, as far as I know, so I guess it’s a moot point.