Tank bangers (the ball on the elastic thing) work alright, but many people have them, and too many times you are underwater and all you can hear are bloody tang bangers twanging away. Does my head in...
Torch signals work if you have a powerful-enough torch to make a visible beam in bright daylight conditions.
A karabiner rapped on your tank also works.
I have a quacker attached between my BCD power inflator and low-pressure inflator hose. I only use it when I need to get my buddy's attention immediately and can't wait 30-40 seconds for them to glance in my direction so I can wave at them. I only give one quick quack and that is enough to alert them to look in my direction. Usually it is if I spot something like a shark or manta or similar, or need them to do a bit of modelling. Not that many people have quackers, so if they hear one, they tend to know it is me.
Mark