It depends on what you want. If you want one book that fills most of your needs (teaches, provides graded practice material, etc.) it’s going to have to be a traditional textbook like Genki. But other than that, here are some other useful resources that I think all fill valuable niches:
A Dictionary of Basic Japanese Grammar: Excellent resource for looking up grammar points you’re hazy or confused on while you’re reading. It’s also great just to keep with you and read a section or two whenever you have a chance for a bite-sized lesson. It has some short essays at the front and glossaries in the back that are also extremely useful.
Japanese the Manga Way: Good as a primer if you haven’t studied grammar at all, lots of people (myself included) say it helped them get the big picture of how Japanese works before moving onto learning the nitty gritty details with other resources. It’s also good as a quick refresher since it’s concise and can be read quickly. This one basically fills the slot of Taekim. It’s just a short, easy to understand overview of basic Japanese grammar that you can read quickly and helps prepare you digest denser materials.
Nihongo no Mori Youtube Channel: Excellent channel, but to be honest I’ve only watched their N2 and N1 videos so I’m just assuming their beginner content is as good. I think this is the best Japanese language channel on youtube, but I have a crush on Takepan so maybe I’m biased.
Making Sense of Japanese: This isn’t really a grammar textbook, it’s just a collection of essays by the eminent Professor Jay Rubin which are intended to help students solidify a lot of fundamental concepts which are typically confusing. I absolutely love this book, but save this one for after you’ve worked through at least one actual grammar book. It’s good for when you’re trying to claw your way out of the beginner level.
Shin Kanzen Master: There are tons of these for pretty much all skill levels and subject (grammar, reading comprehension, vocab, kanji). I’ve only used the N2 and N1 reading comprehension ones but I like them quite a bit. I assume the beginner level ones are also good.
All of those cost money except for Nihongo no Mori. If you’re REALLY pinched for cash then I think the best free, online resource is Imabi, but I suggest you invest in actual books at some point if you’re serious about learning.