First off: Reminders. Depending on what exactly you're after, this app might just fill in your needs.
However, when we talk about task managers, a few of Things' competitors are:
Wunderlist,
2Do,
Todo,
Omnifocus and
Todoist.
I'm still on my queest to use Apple stock apps as much as possible, so I've gone back from Things to Reminders.
Still, one day when I need more, I'll be back with Things in a blink of an eye.
IMHO Things is very user friendly, yet offers the right amount of tools for real task management.
You can do GTD with it on a level that you yourself can set.
And, personally, I love the look of Things.
So yes, to me Things is probably the best out there.
FWIW, I've had rather bad experiences with both
GoodTask and
Calendars 5.
They use Reminders' database and have built an interface around that. As such, it's a great idea.
However, while Reminders allows you to add a due date / time, these two can add a second alarm date / time.
In both cases, my Reminders database got corrupted (perhaps by the addition of that extra date).