There are different forms of B12 available (cyanocobalamin, methylcobalamin, and hydroxocobalamin). Although there are some differences among them, for most people any of these will be effective.
Most importantly, be aware that your body can only efficiently absorb a small amount of B12 within a given period of time. To absorb more B12, divide the dose over the course of the day and take separately from a B12-containing meal. If you take a higher dose, you will absorb only a small amount of what you take -- although this may be appropriate in some situations. For details, see the B12 section of the B Vitamin Supplements Review >>