It's not the question of best for all, 'it's a question of best for your application. There are two main types of a/b switches. Active and passiv. With passiv switches, the signal will simply be switched. I thing any signal loss or change will be a rsult from kabels, jacks, plugs. I thing the main disadvantage of passive switches is, that you sometimes can hear a click, if you switch. Most times this is caused by DC voltage, that comes out of your amp or active bass.
Batteries or DC supplies will only used for the LED. With one exception, i have seen a passive A/B/Y Switch, which uses relais for switching. So here you have to use a supply.
Active switches will do more. Normally you don't hear anything, but some pedals will kill your sound. Active pedal mostly operate with OPAmps and they will change the sound of your bass, in partikular when you're using a passive bass.
I use only a passive switch and i have build my own. There are very good diy kits on this page:
http://www.musikding.de/