Actually, in terms of KR canon, Decade is the most powerful. More specifically in his Fury Form(I know the official name, but this sounds cooler.) Because he was able to single-handedly destroy every Rider. The only reason Kivala won is because Decade let her win.
The previous 9 Heisei Riders never had any "canon" between each other except for possibly Kuuga & Agito, but other than that your point is moot.
Decade may have connected the previous 9 Heisei Riders (and Showa Riders) together, but he never connected any official continuity with all the Heisei Riders. Granted, he did appear with W a couple of times..........but that's because the concept of Kamen Rider Decade allows Decade to "pass-thru" the continuities of other Rider series.
So as a Decade fan, I still say Decade isn't fucking KR canon to anybody but Decade himself. Decade is his own walking "canon" with the ability to interfere in the continuity of any other "canon."
Abilities alone, Decade is the most powerful b/c he can use any Riders' powers (altho it isn't confirmed either, I still believe Decade can use the powers of the Showa Riders too........despite what Kaito says since Kaito has a reputation of lying).
But you can't compare Decade to "KR canon," there is no KR canon before Decade except the Showa Riders.
LOL. Epic trolling.
The answer is Agito, by the way.
For the record, Decade wasn't a choice on this poll...........so I picked Agito instead :thumbs: