What is considered the best sim card in Japan?
I don’t know if that’s really a Japanese (language) question.
Even then, what does “best” mean here? In terms of price? Coverage?
Except if you plan to go to the deep 田舎, coverage isn’t really an issue, so I would just check on 価格 dot com (go to mobile_data/sim/; I’m not linking a commercial site here) to see which one is the cheapest.
Do you have an alien card?
Because if not you have two options:
Give up on a phone number and live only with SIM data plan. (You can get a phone number with Skype or something similar, but not a Japanese one)
Rent the SIM card (I’ve never done this).
I have tried multiple SIMs but the best results the physical ones I had were these from b-mobile:
On my last trip though, I found something better. If your phone is compatible with eSIM (you have a list of compatible devices in the help section):
Oh wow, DEFINATELY going with this next time. Now that we’ve left the US again they’re going to cut off my Google Fi any day now (rip).
To give a basic recommendation I’ve been really happy with Ahamo. Cheapish and 20gb a month, which I’ve never exceeded. Signal has only been an issue a few times in subways. They also use eSIM, which if your phone is cool with that, sets you up without needing to install a physical SIM card. In theory meaning you can keep any foreign number that you have, if you wanted to. Good luck!