I pulled up an excel spreadsheet today to figure out what the best secondary for Dedede would be since certain circumstances have made me unable to use my current secondary, Metaknight, after November 1. I went through a points system that was sort of complex and I won't explain it here, but for the most part I calculated how well the matchup coverage would be against Dedede's bad matchups (except Metaknight). Tier rank of the opponents was also factored into the equation since that often dictates the likelihood of facing them. Here are the trends that I found.
EXCELLENT CHOICES - 5 points or higher
1. Game & Watch (8.79 points)
2. Marth (8.78)
3. Toon Link (5.84)
4. Zelda/Sheik (5.35) (This refers to switching between the two, it is not a tie)
5. ROB (5.13)
GOOD CHOICES - 2 to 5 points
6. Snake (3.25)
7. Zelda (3.05)
8. Pit (2.83)
DECENT CHOICES - -1 to 2 points
9. Falco (-0.28)
10. Ice Climbers (-0.56) & Kirby (-0.56)
METAGAME CALLS - -4 to -1 points
12. Olimar (-1.38)
13. Pikachu (-1.64)
14. Lucario (-1.71)
15. Shiek (-3.38)
16. Peach (-3.53)
Anyone with fewer than 4 points I didn't bother to list, though I guess it's notable to say that Wolf was very close to not getting cut since he had -4.01 points.
One notable thing is that most of the characters on the list who Dedede has bad matchups against often had negative points, with the exception of G&W who ***** all Dedede's bad matchups, especially Pikachu (which is what actually gave him the boost over Marth in the end). I believe that this is largely because they would have neutral matchups against themselves (ie. dittos) and many would also be solid in general but lack an extra boost needed to give them an edge in other important matchups (such as Falco not having a real advantage against Ice Climbers, etc.).
Of course, the data I took was from a matchup chart, so it's probably wildly inaccurate. Still, it's a decent general approach.
Personally, I think I'm going to try Zelda/Sheik since I used Zelda quite a bit when Brawl first came out. I can see myself eventually falling back on Marth though.