As far as first series go, I consider Megaman: Polarity to have done pretty well. It all started with a craptastic little movie, offically called only "Polarity (starring Megaman)" I was amazed when it didn't get "blammed" as Newgrounds calls it, and people actually liked the concept.
A few weeks later, "Megaman: Polarity 2" was released to better, but still critical reviews. I did use better sprites and animation, but the frame rate was still too slow and my attempts at humor didn't help matters.
Over the next three months, I worked on mutilple fight sequences, a good storyline, and a reduction of humor attempts. I really put a lot into this one and started realizing what makes a great Flash work. "Megaman: Polarity 3" hit Newgrounds, gaining it's first daily award.
Polarity didn't end there, however. One user who saw Polarity 2 liked the concept so much he said to try making a game out of it. Makes sense, seeing as how it was a crossover of two other games. About a year later, I started working on it, but didn't know much about making Flash games, so I pretty much learned as I went along. "Megaman Polarity: Reconstruction" was the final result, and I was quite proud of it. It won not only a daily placement award, but a weekly "Review Crew Pick" award.
A while later I started contemplating making another sequel to the series, a second game called "Megaman Polarity X." However, it's in "development hell" right now and I'm considering the project dead at this point. I "went out with a bang" with the first Polarity game, so this one had better live up to it's predecessor if I ever decide to take on the project again.