I use NGINX because it’s what I’m familiar with. If I was starting again, I would probably use Caddy.

Using ‘hours of use’ as the metric, it would be Plex. The ones I use every day are Libreddit, TT-RSS, Huginn and Reddit-RSS - and my own journalling app and pocket clone.

Try a delivery test to an Outlook / Exchange server. I’ll be amazed if it goes through.