The easiest of the 7 1/2 habits for me would be play, of course...although I've found that as I get older it gets more difficult to drop all that "important" stuff I'm supposed to do and find time to play. That's something I need to watch out for. When the laundry becomes more important than having fun, you may be in trouble.
The most difficult of the habits is probably #7. Teaching and training is not my forte--I tend to jump all over the place and be just generally scattered. Which is why I am now a librarian, and not subjecting classes of freshman to long, rambling, disorganized lectures on the works of Flannery O'Connor. I really have to work at staying focused, and I need lots of feedback from those I'm working with to make sure I haven't skipped over or shortchanged anything important.