Which Vinaya has the least number of rules?
Of the extant Vinayas, the Mahasanghika has the least rules, if by "rules" you mean "patimokkha rules". 

As per your post, the difference is almost entirely in the number of sekhiya rules, which are essentially table manners and the like. Indeed, in the early sources the number "150" rules is mentioned, and I believe this means that patimokkha in the time of the Buddha did not include the sekhiya rules. So if we were to count the rules as per the "original" patimokkha, there would be almost no difference between the Vinayas.

The patimokkha rules, however, are only a small, albeit important, collection of rules. Throughout the Vinaya there are many minor and incidental rules and procedures. Since these often occur by way of permutations of factors for other rules, it is difficult to count them exactly, and I don't believe anyone has done so.

The fact that the Mahasanghika has fewer sekhiya rules has been misinterpreted to imply that the Mahasanghikas were lax in Vinaya, and that their Vinaya text is early. In fact, from this we can't really draw any conclusions about the Vinaya practice of the Mahasanghikas (since the details that differ deal with tiny matters of etiquette), nor about the historical development of the Vinaya texts (since in other aspects the Mahasanghika Vinaya appears later than the Theravada).