The first weekly challenge was to complete 9 missions on Heroic.  I wanted to play through the game on Normal first, and take my time with it, so I didn't really give this challenge much of a try.  But, since then I've completed every weekly challenge and it would be fun to see how long I could go before I missed a week.
 
So far, I think this week's challenge is probably the most *erm* challenging, depending on which hoppers and gametypes you play most.  I nabbed it on the first day, but I was specifically playing just for 30 kills in a single match.
 
50 rounds of firefight was the most time consuming, but not difficult to do.
 
I don't like the weekly challenges that must be achieved in a single day or a single match.  That should be a daily challenge, not a weekly challenge.  So far we've had one to complete all of the daily challenges (got that one the first day) and the aforementioned challenge to  get 30 kills in a single match.  You could have unlocked that weekly challenge in the first 15 minutes of playing on Monday.  (It took me 4 matches IIRC, and that's because standard slayer matches were voted until my 4th Rumble Pit match.)