The acoustic guitar may be the world's most widely-used and best-known instrument - and with all that popularity comes a huge range of accessories designed to help it handle all kinds of different performance and recording situations. Acoustic guitar pickups are definitely some of the top picks for guitarists who want to take their instrument further, and they'll allow you to make any standard acoustic into an acoustic-electric instrument that's ready to play just about anything, anywhere. Connect to an amplifier to reach the furthest audience members in a big venue, plug into a backend sound system, or feed your signal into effects units to shape your sound: the options go on and on.
Acoustic guitar pickups come in a few different varieties. The most straightforward models are the ones that mount in or around the soundhole, which include simple microphone-style pickups like the LR Baggs Lyric Classical Acoustic Guitar Microphone, as well as single-coil and humbucking pickups similar to the ones you would find on an electric guitar. The Fishman Neo-D02 Humbucker Soundhole Pickup and the Seymour Duncan Woody SC Single-Coil Soundhole pickup are two of the best-selling examples of this type, and there's a whole lot more where they come from!
If you'd prefer to put your pickup somewhere other than the soundhole, one type worth looking at is the transducer pickup, such as the Dean Markley Artists Transducer. These respond to the vibration of the guitar's top rather than the strings, and you can stick them to different places on the top to emphasize different sounds. Or, if you don't mind doing a little more in-depth tinkering with the instrument, you might want to check out piezo pickups like the Fishman Saddle Acoustic Guitar Pickup or the Shadow Quick Mount Classical Guitar Under Saddle Pickup. These models are a little bit less "plug-and-play," but their great performance has made them somewhat of a gold standard for acoustic guitar pickups.
Since each type of pickup, like each guitar, has its own quirks and characteristics, there's no single model that could be called the best. It depends on your personal tastes, the guitar it's going on, and the sort of music you play - in fact, you might even prefer to mount multiple pickups. This would give you the choice of using different ones for different scenarios, or even using multiple pickups mixed together, which can create a nicely-rounded sound taking advantage of the best qualities of each one. As with guitar strings, a little experimentation with acoustic guitar pickups can go a long way!