No, its not some new phone from Nothing or the newest iPhone. Everybody is wondering what it is, but few realize they already own it. You probably don't want to hear this, but the best least addictive phone is the phone you're using right now! I don't mean current phone in your current setup. What I mean is you must turn on greyscale on your phone screen and turn off all notifications. There are exceptions to the notifications rule but doing these two things will quickly curb your addiction to your phone.
Turning greyscale is completely change the way you view your phone. Gone will be this magical time sucking device. Everything that was color is now black, white and grey. It will take some getting use to, but you will quickly realize how boring seeing at different shades of grey will be. You'll slowly only check your phone when you need to.
In the beginning, notifications on smartphones were used to notify you of actual things. Slowly, it has turned into notifying you of irrelevant things like "remember to write that review!" Because most notifications aren't actually important, you must also turning off notifications on your phone. I don't mean turn off all notifications though. You can leave notifications on that are from actual people trying to contact you (ex. phone calls, text messages). Even group chats notifications should be turned off unless you know most of the group. Email notifications should be turned off as well as they don't require your immediate attention.
By simply doing these two things, you've now got yourself the least addictive smartphone in the world. If you can stick to it, I can guarantee your addiction will be cured and you will wonder why you were ever addicted it to begin with.