For transactional emails, notifications, reminders, etc, I want to make it so writers/designers can set up the emails and maintain them, and then dynamically insert fields, that I then replace when actually sending the mail from code.
I think the ability to use a basic layout template across individual email templates would make things a lot easier (think header, footer, standard typography, etc).
What is best for this? Why would you prefer Mailgun, SendGrid, Mandrill or something else?
If you need your emails to be sent in a time-sensitive manner, I'd recommend SendGrid. We were using Mailgun and the lag because they aren't "transactional" in nature caused issues for us. SendGrid also has the ability to do dynamic templates and bulk send from their API. I don't know that they have the shared layout ability you mentioned, though.
The only transactional email service that I've been able to stomach is Postmark! It is by far the easiest (and quickest to get feedback from) service that I have come across. While drowning in attempts to debug Mandril, Mailgun and others I get quick feedback from Postmark in what I need to do.
Postmark for the win!