There are many different USB charger in the whole market, and bellowing is some import point when choosing the best USB charger.
Voltage /Current/Power: If you only need charge a ordinary small electronics devices. Maybe an ordinary 5V 1A USB charger or 5V 2A charger is OK. But if your cellphone or tablet is USB C port or supports a Quick charger. A quick charger or USB-C charger will be more suitable.
Style/shape: If you want to put the charger into pocket, small one and with a foldable plug will be the perfect one. If home use, a larger one with several ports maybe more convenient as you can charger you cellphone, tablet or other electronics device at the same time.
Safety remarks: Different market require different certifications, if you are in USA , remember to choose the charger with UL mark. And if you are in Europe, CE is the guarantee of the quality. KC is for Korea charger, PSE is for Japan charger and SAA RCM is for Australia charger.