This is an area that is sort of like asking "who has the prettiest girlfriend." There are a lot of choices and a lot of varying opinions. I know of about 30 different backup programs for OS X (I can give you a list, if you like), and there are a bunch of different philosophies about what is the best backup plan.
You might find this e-book very helpful to explain this very complex area:
Take Control of Mac OS X Backups
$15; free sample available
OS X 10.5 includes Time Machine:
These two cloning programs are both very popular for creating a bootable clone backup:
Carbon Copy Cloner (donation requested)
SuperDuper ($28 to unlock all features)
A lot depends on how much data you have to backup, how important is your data (i.e. is your data part of a business), how much time are you willing to invest in backing up your data regularly, what is your budget, etc.