Which SMS provider would you recommend?
I guess its based on which provider you choose based on the budget or sms gateway fees. There few extension provided by Organization that integrates with SMS gateway

Clickatell
Twilio
Message Media
My sms myntra

You may also find more if you try to search for SMS gateway name with CiviCRM.
We have few clients using Twilio and have positive response so far, few others use different providers because of budget constraint and SMS charges, so we have implemented extension to support that(very simple to integrate).