What is the best type of exercise for someone with lower back pain?

Also, I gave this answer to a previous similar question.

For these types of clients the sequence of starting an exercise program should be:
1. Physician Clearance.
2. ROM assessment and any other assessments that are indicated.
3. Begin with foundation movements at all joints within comfort.
4. If available, include aquatic exercise.
5. Begin addition of resistance.
6. Use slow careful progression/regression as tolerated.

Even my collegiate athlete clients go through this initial phase. The information that I get from this system is the basis for the program design.