It really depends on what type of card you like to make or whether you're just up for any challenge.
All the main challenges are linked on the home page here:
Splitcoaststampers.com
And there's a sub-forum for each of them in the Challenge section here, plus some other monthly ones and others organised by members:
Splitcoast Challenges Forums: Daily Challenges Throughout the Week
I think my favourites are the Ways to Use it, Free for All and Inspiration - that's Thursday,Friday, Saturday. But everyone has their own favourites depending on their own personal style.
Challenge
