Please provide name of best notification services for android and ios

Amazon SNS and OneSignal are mostly used and recommended. You can compare pricing and features based on your requirements. Additionally, you can use trial version to evaluate.

I personally haven't tried OneSignal and SNS. OneSignal is having its own services to send notification, you can avail their trial service, if it suits your requirements then you can proceed further.

I think amazon SNS and OneSignal also used firebase service to send notification.

ok..google is not guaranty to be deleievered but any other service like Amazon SNS or one signal will provide notification guaranty.

Guarantee depends upon Android/OS system and various factors, they are build to send information to avoid spamming. So Google is improving it day by day, the ultimate goal is to provide less clutter and better user experience. Even firebase has written in their note section that it is not guaranteed to be delivered.