There are many free worksheets available, especially online, but still the best worksheet is one that you personally draft. This way, you are able to match the level of difficulty of the activity in accordance to the performance level of your own students. It is not bad to reuse worksheets for another batch of students, but once in a while it is also better to vary the activities you give to kids. Worksheets can be made for fun if it is attuned to the current interests of kids. The kids will respond better to activities close to their own interests.
There are many kinds of worksheets available but the best one is still the personally drafted one. This way one can easily check the level of difficulty that is offered to the kids according to their age and even check the progress level. Worksheets can be reused but it is always better to upgrade them from time to time.