I wrote 2, but honestly that's part guess-work. It's been so long since I've played them that I don't REALLY remember which was my favorite. What I did was run by
http://www.gamefaqs.com and look at the boss lists for all the games, then I tried to remember which ones I liked the best all around.
So I THINK it was MegaMan 2. But I'm not SURE.