What is the most reliable type of emails to receive form submissions?

Would it be gmail, yahoo, a server based email with outlook, mozilla thunderbird?

Which is it less likely to go to spam?

Does email forwarding or an email forwarding cause issues with these forms not going through or ending up in the spam folder?

All of them are reliable, you need to make sure that your notifications are properly setup, as shown on this guide: https://www.jotform.com/help/25-Setting-Up-Email-Notifications 

You can check if the recipients are in bounce list: https://www.jotform.com/help/262-How-to-Remove-Your-Email-Address-from-Bounce-List 

Also, you may white-list our domain names as we use Amazon SES method for "noreply@jotform.com" sender:

·        jotform.com

·        email.amazonses.com

·        amazonses.com

·        jotform.co

·        secure.jotform.co

·        s3.amazonaws.com  

·        static-interlogyllc.netdna-ssl.com

If you do not want our email addresses  noreply@jotform.com or noreply@formresponse.com  to be setup as the Sender E-mail , you need to white-list our Jotmail IP addresses within your email server (or ask for it to your email service provider). Read this article for more details: https://www.jotform.com/help/145-Whitelisting-JotMails-IP-Addresses 

Let us know if you need more help, we will be glad to assist you.