The best drum pedal is unique to the individual drummer. First establish a budget, then try all of the drum pedals in that price range to find one that feels comfortable to play. Pedal adjustability is important, so look for a drum pedal that can be adjusted to suit personal taste and playing style. Generally, the higher the cost of the pedal the better quality materials that went into its construction, however, a playable drum pedal can be had at almost any price point.
The most common configurations for bass drum pedals are the single and double bass pedals. In the case of double bass pedals, there are two beaters on one pedal, called the master pedal, and a second pedal without beaters that's called the slave pedal. The action of the slave pedal is often different from the master pedal, but the difference shouldn't be so great that the drummer has to work harder to play because this can cause leg fatigue and timing issues.
Once the preferred configuration has been established, the next step is to choose the type of drum pedal: strap or belt drive, chain drive, or direct drive, which is also called solid shaft. Each design works well and the best would be the one that feels good to the individual drummer. Strap drive has a piece of flexible material connecting the pedal's footboard to the beater. The strap can be made of a variety of flexible yet sturdy materials. Strap drive pedals are quieter than chain drive and have a lighter feel.
Drummers who favor a solid, heavier feel will often prefer chain drive. A chain driven drum pedal has either a single or a double chain connecting the pedal's footboard to the beater. Higher end drum pedals typically use a double chain for more stability and a smoother feel. A direct drive drum pedal has a solid piece of metal with special fittings connecting the footboard to the drum beater. This design is very smooth and responsive and is favored by drummers who need both speed and accuracy.
Most drum pedals use a standard drum key to make adjustments. For the ones that require a specially designed key, consider investing in one or two spares in case one gets lost or broken. Exactly how many adjustments can be made to the pedal will vary from product to product, but some important basic adjustments are beater angle, spring tension, and pedal angle.