It varies from day to day for me. I set my Move goal too high once or twice and had a hard time hitting it, so readjusted downwards and then had no trouble. On very sedentary days where I didn't leave the house, I've had to make an effort to fill the Exercise ring, though on average it's probably the easiest to fill. I'm a super fast walker, so even an hour's worth of running errands tends to fill my Exercise one.
The Stand one is definitely the easiest to miss, though-- all it takes is to sleep for 13 hours and there's nothing at all you can do in the remaining 11 to check off that day! If you had trouble with with Move or Exercise on a given day, you can fill them up fairly quickly with a brisk walk or bike ride, but once you miss 13 hours on the Stand goal, you're toast. I have a pretty terrible sleep schedule, so it's not unusual for me to be up for over 24 hours, then crash for 15 or 16 hours afterwards. This really messes up my Stand activity, though, so I've been trying not to do that since I got the Watch! I think my current Stand record is 22 or 23 hours.
It's almost 4pm here in Ireland. This morning I got up early, then went to the mall to get groceries, and I'm now at 310/300 Move, 26/30 Exercise, and 11/12 Stand. To be honest, with the exception of oversleeping for the Stand one, I don't struggle to fill my circles. Only on very lazy days do I find it difficult, though I think my Move goal is a lot lower than other people's!