Isn't Both Zeed and Darkness Bagramon are fused digimon? They don't count I think.It's definitely Zeed in general, almost no questions asked. He has too much hax, is way stronger than the sovereign and the RKs, and can casually destroy and create realities. There's a reason he's nigh impossible to been in vsbattles forums... He was also unable to be destroyed by whomever sealed him--since he was just sealed. Whether that was God or Shakamon, or someone else, Unchained Zeed is higher than them. The 7GDLs are multidimensional constructs in their true forms too, so Ogudomon's "true form"--if he has one--would probably be second strongest. However, both are technically fused.
As for the anime, Apocalymon was going to destroy two dimensions with his suicide, but DarknessBagramon was going to affect "all worlds" (correct me if I'm wrong on that). Quartzmon isn't really in the same league, considering how sure Bagramon was about beating him with the Brave Snatcher (this was before they ever mentioned a "root").
Half of your list are fused digimon, the question is the strongest unfused digimon.1. Shoutmon X7 Superior Mode. Come on, golden boy has to be on top.
2. Bagramon, by extension.
5. Alphamon/Omnimon (since it's still a no-score draw between them)
6. The Burst Modes
7. Crimson Mode (it's hard to judge where he falls; his only opponents were Agents who don't fall on the scale, or easily one-shotteable bugs in the movie. He's still the strongest non D-Reaper thing in Tamers, so he gets a spot)
8. Rest of the RKs (Frontier/Data Squad.)
10. Daemon (wasn't sure whether to put him above Apocalymon since he left undefeated, but that whole arc was a mess so it's hard to judge. Besides, he can't blow up the universe as far as we know. The one who can blow up the universe gets ninth place.)
If we go non-anime, I like to think Superior Mode still takes the top spot. Don't care enough to put together a list, but him being made up of every other Digimon (except, we assume, the really special ones) is basically the "infinity plus one" of childhood monster fighting arguments.