Personally, I have been wondering this a lot myself recently. I tried a lot of people mentioned in here over the past month or so and I have to say it looks like I'm going to probably go with Diddy in the end. For myself, he seems to be the best option out of everyone. I had a difficult time picking up several of the characters, but Diddy was fairly natural to me. Plus two of my friends main Diddy (one being very good with him) so I have the people to help me get to where I wanna go.
I think that Falco does not have to have a secondary, although it is certainly quite helpful for people who aren't top Falcos. I'm a newer competitive player myself (attending my first tournament tomorrow in Montgomery ^_^ ) but I can see how a good Falco doesn't need a backup. There are quite a few frustrating matchups for people who haven't put enough time into the game or have enoug MU experience. Myself being one of them, Diddy is going to be useful to me in the future.
Also I would like to note that I find the suggestion of Peach as a potential backup quite interesting. I may have to entertain that idea and give Peach a shot....
