I've never been to college but I have studied military history on my own for the better part of 15 years as my hobby. I read every single night and have learned a lot. I find it difficult to believe the only agenda or guideline for this debate is 1 sentence? "Stalin, the greatest Russian leader of all time?" Maybe I am wrong and the teacher wants ppl to be creative but are there no other instructions or clarification?
The reason I ask is because the question is in such a general statement. There are many ways to measure "greatest" and without some form of reference it might make the task more difficult. Greatest Military Leader? Economic and Domestic leader?, etc.
That said, I would think that there has to be a clear case for Stalin being in the discussion, if not the clear choice. Perhaps the main accomplishment you can set your research/debate around could be that he did lead the country from the brink of defeat to a world super power. Naturally there is a lot more involved in that statement but saying he wasn't a great leader for killing 60 million people is not really focusing on the entire argument.
Great leaders can come in many forms and dispositions, being labeled as an "evil man" does not automatically make someone a "bad leader". They all sanction acts that one would consider "evil" to further their own goals and ambitions but the ability to lead a country (good or bad) is the main question.
----For example, how many Roman Emperors are hailed as great leaders? Under each of their rule, untold numbers of slaves were killed throughout the empire for any reason their owners wished. Those deaths were "legal" for the time but do those deaths make the Emperor a bad leader? It is a complex argument/debate and probably why your teacher is making you do it.
edit: waiting for Marxist dissertation on this subject :)