I have been using three different apps for my todos, email, and calendar and have always desired to have them all in one place. After Sunrise announced they were folding, I looked endlessly for an app that had a calendar that could match Sunrise, a email server that could match exchange, and a todo that was as powerful as Todoist. After some tweaks this app has far exceeded my expectations.
Glad in God about Handle: GTD To-do List and Calendar Management, v1.3.5