Best electric? I don't know, but I really like my kit I got from Spookytooth.
The laws are usually much more lenient in favor of electric bicycles. Some times you might have problems getting ticketed on a gas bike, but you will probably not be bothered on an electric (I have one of each). Here in Orange County, California, there are even rules specifically allowing electric bicycles, while the rules for gas bikes are best described as "gray area".
I built an electric bike from a Spookytooth kit, but I got the batteries somewhere else, so I'm not sure about that. If you shop around, you might be able to find a set of four 17 Ah batteries for about $100. I recommend buying them locally if you can, because they are heavy, and the shipping will be significant.
As for the folding bikes, I have no experience with those.
Hope this info helps.
By the way, Spookytooth does not have many electric kits left, so if you want one, you may have to make up your mind soon.