Kotkin's first volume on Stalin is a far better work that I'd recommend as far as biographies go. Kotkin is very obviously an anti-communist, but even a turbo Stalinite like Grover Furr finds few academic faults with that particular work. The other volumes are less stellar though.

There's also the recently authorized re-translation of Stalin: History and Critique of a Black Legend by Demenico Losurdo which has a free PDF available. It offers insight on a perspective of Stalin that seeks to de-mythologize the "monster."

As for Montefiore and authors of his ilk, I wouldn't rely too much on narratives spun by pop history writers and journalists.