Joseph Stalin was, for most of his life, a member of the Russian Social Democratic Labour Party (Bolshevik faction) (RSDLP(b)) and its precursor (RSDLP) and inheritor (Communist Party of the Soviet Union, CPSU) parties. He was singularly responsible for that party's own history of itself, revised from 1937-1956, History of the Communist Party of the Soviet Union (Bolsheviks): Short Course.
Your question then comes down to two issues:
To the extent that Stalin dictated the criteria of being a good Bolshevik, and that these criteria were contested by old Bolsheviks (notably Trotsky and Bukharin) we ought properly to consider that Stalin's quality as a Bolshevik is "contested." Either he defined the party standards of conduct, in more than simply a nominal and political manner, or he failed to live up to transhistorical standards of conduct defined sometime before 1924.
The extent to which the Bolsheviks were a party that sought to and attempted to advance proletarian self-emancipation in real history is contested. That is: it is contested whether the Bolsheviks were a communist party. Much of this debate rests on questions of Lenin's politically unique position on the relationship between party and class. The communist credentials of most other potentially communist groups (IWW, CNT/FAI, etc.) are equally contestable. However, this is a debate about ideology (what constitutes advancing proletarian self-emancipation), not practice. And we can skip it.
Without needing to resolve the issue of Lenin on strategy, both the NEP and Five Year Plans were a permanent accomodation to the Party controlling the commanding heights of a value-form society, one where despite protestations, commodity and wage-labour confronted each other as capital and working class. That is: the critical elements of a capitalist society, according to Marx's analysis, existed in the Soviet Union. To the extent that wages, capital accumulation and commodity production existed in the Soviet Union, and Stalin bore significant administrative responsibility for this, no, Stalin was not a communist.
Did the Bolsheviks maintain as great a reserve of working class strength as possible through this? We must accept that the Soviet Union was not communist, however, it may not have been possible to develop communism in the Soviet Union. In such a case a communist movement will attempt to best develop the independent strength of the working class. Did the CPSU make this attempt? The answer is strictly no: the party was a cross-class institution and class only institutions were suborned to the party. Additionally the party had a shortage of, and then destroyed, the revolutionary intelligentsia which "stood in for" a proletarian vanguard within the party. To the extent that developing Communism may have been impossible in the Soviet Union, Stalin was significantly administratively responsible for the repression of reserves of working class political, economic and social strength, and so was not communist.
Please do note that Stalin was part of an administrative and political group who bore collective responsibility for their efforts in these areas. He is not communist in the manner in which all of his ilk were not communist—these are not specific attacks on Stalin, but on the ruling clique which he participated in. While Stalin was an able administrator, it took an entire party and nomenklatura to enact the shifting bolshevik platform.