The question of which are the best hearing aids, may necessarily boil down to what is the best one for you, given the kind of hearing impairment that you have and your budget. Hearing impairment may differ from one person to the next; it may come as a generalized decrease in the over-all sense of hearing, it may come as reduction of sensitivity for certain frequencies or it may be a combination of both.
The cheapest and best hearing aids for people with a low budget are analog hearing aids. Analog hearing aids are miniature amplifiers which work to remedy hearing deficiency by increasing the volume of the sound. The disadvantage of analog sound signals is that the sound produced tends be "noisy" or have electromagnetic disturbances due to the physical medium producing the sound. The sound produced by an analog hearing aid is therefore raspy, much like an intercom. However, if your budget isn't high, they will serve your purpose.
If money is not a concern, the best hearing aids will be digital hearing aids which are fitted with analog-digital-analog converters. We can consider digital as the conversion of the sound wave of the analog signal to intervals of binary values representing the different properties of the analog signal at a certain point in the sound continuum. Because the interval between the sampled points is very small, the sound produced by digital converters is heard as a continuous stream. Digital sound may be edited to eliminate "noise" or to put special effects into the recording. After they refine the digital signals, the values are converted back to analog sound before it can be heard. Digital hearing aids have a central component that works like a sound mixer.
The best hearing aids are evidently digital, and they are also more expensive than analog ones. However you have more flexibility in adjusting the sound. You may increase or decrease the volume, as well as mix the sound in any way you want, whether to make the higher or lower frequencies more prominent or to muffle the sound.
According to where you place them, there are 4 types of hearing aids. The best hearing aid for people with only a slight impairment of hearing will be the smaller models made to fit "completely in the canal (CIC)" of the ear or just partially "in the canal (ITC)." Their small size limits the size of the battery and the amplifier and therefore these models are best suited for mild hearing loss. A slightly larger version of the ITC is capable of directional focusing.
For people with more serious hearing impairment, the bigger models are more suitable. The "in the ear" (ITE) and the "behind the ear" (BTE) models have larger batteries and amplifiers and are therefore capable of producing a louder reproduction of the sound.
Finally we have the "open fit" model which leaves the ear canal open. The device is attached behind the ear and a slender tube with a speaker at its tip that is inserted into the ear canal. Because it does not obstruct the ear passages, this hearing aid is ideal for high frequency hearing impairment.