Japan has a few MNOs and a few MVNOs. As a foreigner (assumably, an English speaking one primarily), I do have some advice for what you should and shouldn't get.
For MNOs, order of preferences based on speed, coverage, in Tokyo etc:
au (KDDI)
SoftBank?
NTT docomo (has the best nationwide coverage, along with au, for users in rural areas)
Rakuten
Specific notes:
au is by far the fastest carrier I've used in Tokyo, but it feels like a step-behind in terms of administration. You can transfer SIM cards overseas, but you need to schedule it in the My au app first. The biggest issue I've had with au is that it seemingly is impossible to update your credit card to a different one, if your name 'doesn't match' your contract name. While this might not seem like it's a big deal, none of my US credit cards, Japanese debit cards, or Japanese credit card (AmEx) were suitable to migrate my au payment to. I ended up porting one of my numbers away from au for this exact reason. au is also typically one of the most expensive.
SoftBank does not permit me to be a customer with them. I'm on the business manager visa, and apparently SoftBank doesn't seem fit to give me a contract. They reject me with a “status of residence” issue. The problem is that fundamentally, I love SoftBank. If you can get a contract with them, they have very good coverage, and an unbeatable overseas roaming plan with unlimited data use.
NTT docomo is a great carrier! I recommend you use the “ahamo” sub-brand, which is online only. ahamo approved me for use within the first four days I lived in Japan with only a 4 month residence card. NTT docomo is also the most seamless in terms of switching SIM cards overseas (“it just works”). I've seen reports of issues with their 5G service, but I have not experienced these issues. They have probably the widest theoretical coverage in the country.
Rakuten is the newest MNO. If you have a modern phone with good band support, Rakuten also has an English friendly user interface for signup and management. Rakuten also sends emails with English at the bottom, which is nice. Rakuten is also the cheapest! But it's possible to spend a lot of money with Rakuten if you add on services like voicemail and call plans.
NTT docomo doesn't really have MVNOs that are official sub-brands. They have ahamo, eximo, and irumo. Ahamo plans are online-only administered. Ahamo is specifically different, because you cannot use a d garden or many of their in-store services. You're expected to do everything through the ahamo app. But it has the simplest billing structure with no frills and only a couple addons. Eximo is their “normal” plan for normal use with in-store services. It's more expensive. irumo is the budget brand with basically no data. You probably don't want irumo.
SoftBank has LINEMO and Y! Mobile. I haven't had experiences with these MVNOs. I don't know if their MNVOs would reject you like I was rejected from SoftBank proper. SoftBank and PayPay have good bonuses tied together.
au has povo 1.0 and 2.0. I don't have experiences with them but some friends have used povo 1.0 quite happily.
Rakuten doesn't have MVNOs!
Mobal is pretty much the de-facto foreigner friendly cell service. It is comparably cheap, but there is a big caveat: it has no overseas support. Which MNO backs you will be up to which type of SIM card you get. The eSIMs are issued by NTT docomo. There are physical SIMs floating around from SoftBank and au, as far as I'm aware. You cannot migrate between them.
With the exception of Mobal, who really doesn't care, all of these services expect you to be a resident of Japan. Thanks to Mobile Number Portability (MNP), you can freely switch between carriers quite easily. If you're bringing an overseas phone without the Technical Conformity Mark, technically you're in unsupported territory. In-practice, modern iPhones and Android phones will work fine on domestic networks as long as they aren't carrier locked or financed in your home country. If you have an older phone, you might suffer from poor band support. Rakuten is the only carrier where this is a huge concern, because their network heavily depends on new 5G bands. If you end up in this situation, you may have poor speeds or coverage on the fallback bands.
My preference/suggestion for any new entrant into Japan is:
Rakuten Mobile to save money, best English support and experience hands down.
NTT docomo for general coverage and ease-of-management with the ahamo app.
SoftBank if they approve you, you want to use PayPay a lot, and/or you want to roam internationally a lot without excess charges.
au if you demand the fastest speeds, at the expense of cost and account-management annoyance.
I personally have contracts active on:
Rakuten Mobile
NTT docomo
I have used all MNOs.