Budget is a big factor. Chorus pedals have a wide range of prices from the lower end Joyo at $35 to the boutique Analog Man at $234. Decide your budget before looking any further. Give yourself some wiggle room if possible in case you find something that is perfect, but a little outside of your price range.
Features are the next to look at. Some of the best chorus pedals have both multiple features, or one single knob. It all depends on you the player. If you like simplicity, go with a pedal that has minimal settings such as the EHX Small Clone. If you like to tweak your tone to the high heavens, go for something like the Digitech Hardwire which has three knobs to edit seven different chorus types.
Analog vs Digital
In the good 'ol days, most chorus pedals were strictly analog. Newer technology now allows manufacturers to offer both analog and digital chorus pedals. Some of the main differences between the two are how they each process the signal and how they go about altering the sound.
An analog chorus pedal will work directly with the signal while a digital chorus pedal must convert the analog sound waves to a digital format, complete the signal processing effects, and then send the digital signal back over into an analog sound wave.
Due to the different ways analog and digital chorus pedals process the signal, this can cause the two to sound a bit differently. An easy rule of thumb to remember is that a digital chorus pedal gives you that "bright" and crisp sound while an analog chorus pedal produces a fat and "warm" tone.
A chorus pedal can be integrated into all types and styles of guitar playing. Whether you play blues or technical death metal, you can still use a chorus pedal. Keeping analog vs digital in mind, if you're a shredhead that aggressively attacks notes during a solo you would want to go for a digital chorus since that will give you a crisp response. If you're playing a warm, fat, juicy jazzy/bluesy lead or a clean strum, you would want to opt for the warmer analog chorus to give you a good sound.
When looking for the best chorus pedal, make sure to test if you have that opportunity. Guitar center or local music shops carry a wide variety of different brands so you should have several chorus pedals to try out. Try to bring your own guitar and find an amp similar to yours to get a true idea of what the chorus pedal will sound like through your rig.
If you don't have the luxury of testing, check out reviews at online guitar shops and watch videos on youtube. You can also ask on forums or check out a list of the best chorus pedals.