They come in many different kit variations. 14-45, 14-42, 14-140 and 20mm are all possible combinations.
The G series started off with the 14-45, then went to the newer (and nastier) 14-42, the GF was available with 20mm and 14-45/2 IIRC and the GH were 14-140, I think early GH1s could have 14-45 too.
I have the 14-45 on my GH1 but I bought it body only (originally I had an m.Zuiko 14-42 mk1 on there, but it was terrible).
As to which one is best, depends what camera you're looking at and what you want to do with it.