There are a few that continue to amaze me but I'd have to go with Calendar. I can make entries on any device - iPhone, iPad, MacBook etc. and they instantly appear on all the devices I own. Having had a lot of hospital appointments in the last 6 months, I feel safe knowing how reliable Calendar is.
There are few things I enjoy in life, but one thing I do really enjoy is that feeling I get when I find an app that is well developed, and practical, which I can use as a daily tool. There are so many Apps that are nicely developed but aren't very practical. More gimmick than useful. The iOS calendar is very useful and practical. While I use OmniFocus I also use Reminders and Calendar.