Just as the title asks: what was your favorite season of Sailor Moon? Why was it your favorite?
Favorite: Of the four that I have seen, my favorite was Sailor Moon R. Not having ever seen Stars, I can only pick from the other four.
Why: For one, you got a little bit of a bonus in that R had two story arcs. Admittedly, the first one was very short and filler to allow Takeuchi to catch up, but still, it was a nice bonus.
Secondly, I enjoyed the multilayered antagonists and the glimpse into the future lives of Mamoru and Usagi. I thought that the tension between Mamoru and Usagi was a bit contrived, but otherwise, I thought that R was very thoughtfully written and well executed. The plot was a little more complex and we saw some character development as well, particularly of Usagi. It also helped that I actually liked the villains as villains, something that is not always the case in other anime series that I have seen (Sailor Moon has been very consistent in presenting strong villains, which is one of the things that drew me to the show).