Rav Moshe had several reasons to hold that pot was assur. Search on Mi Yodea and you’ll find detailed description and a link to his teshuvah.
I’m not aware of any Rav who has said it’s permissible.
Regardless, one shouldn’t smoke marijuana. For one thing, it’s a big risk, because a large proportion of people experience negative side effects like paranoia and panic attacks, even people who never realized they were predisposed to such mental disorders. And these things (like panic attacks) can persist after one has stopped using the drugs as well. Also, there’s growing evidence marijuana makes certain people come down with major, permanent, life-changing psychiatric diseases like psychosis and schitzophrenia.