“Is the last rebbe considered greater than all the previous Lubavitcher Rebbes, that the last rebbe is considered Moshiach (by some) but the all the previous Lubavitcher Rebbes are not considered to be Moshiach? “
In every generation there is someone worthy of being moshiach. I don’t see anything wrong with postulating that perhaps your rebbi is this special person. I assume that the Lubovichers considered all of their Rebbeim worthy of being moshiach in their generations and I have no problem with that. I don’t even have a problem with the last one being called moshiach while he was alive. But just as the previous Rebbe’s ceased being referred to as moshiach after their petira, so too they should have said that once the last Rebbe died, whether or not he was worthy to be moshiach in his lifetime, he is now definitely no longer moshiach. The fact that they continue calling him moshiach shows that they feel he is greater than his predecessors.