"The Best" is mostly determined by several factors, just like asking the general question of what is the best car. What accordion is this to be installed in and more importantly, what options do you need/want and is your budget?
In terms of brands, likely the Limex system is known as being one of the better ones out there, however, as an example, to install the complete midi kit with mics for the acoustic accordion, on a full size accordion, one can spend upwards of $5000US, and add even more to that if you wanted wireless options. One could also go bargain basement hunting and likely find systems in the $1000-$1500US range as well.
At these price points, it is within reason to find a used Roland V-accordion or an older acoustic accordion with the midi already installed instead of installing it on an acoustic accordion.