kamen ride-o!

Tendou is no better, but it's all about presentation in this case. Tsukasa is supposed to be fallible, but it's so rarely shown it feels like a flaw that was shoehorned in.
His behavior is mean spirited and rude, but others see it as "Tough love" or him just being himself, while those who are against it are seen as just not understanding him. Generally, his dickishness is treated as a charming quality. His abilities are inconsistent, most jarringly being able to use powers without changing forms just to show us how "Badass" Decade apparently is.

They're both sues in their own right. While Tsukasa isn't as bad as Tendou when it comes to performing tasks, it just doesn't feel like he's any less weaker.

Fair enough.

"Power leves" in tokusatsu are always iffy since these are merchandise driven shows. A good example would be in Gaim when Ichigo arms first appears. Even though ryugen is already ranged, the only reason Kouta and michy win is presented to us as ichigo arms

As for the whole Tendou/ Tsukasa/ Mary Sue-thing, I'd say Tendou perfectly fits a possible Sue-definition as he is in fact a character, 99,9% flawless and so on and Kabuto overall feels a lot like a fanfiction.
As for Tsukasa, you could call him a Mary Sue, but I'd say he's even less than that. Tsukasa was not a character, he was a placeholder for whatever shit the authors felt like doing with him. There were episodes were he was an awful person (like the Blade world, with him constantly picking on Kendate and than indirectly admitting he was having fun doing so) and other episodes where he was not that bad at all (I'd say he was kinda okay in the Shinkenger episodes, for example). He never had a really consistent character, nor development because he acted different in every single episode.

Exactly. His character arc never made sense to me at all. The entire series gave me the impression that he was a dick, then all of a sudden in the movie, he's the nicest guy imaginable.

Tendou is a sue, no doubt.
But that's fair (Also, the Blade world pissed me off so much, especially after "Yuuseless" basically saved the day but Tsukasa still got to give the important speech in the end).

Which person from a kamen rider series do you miss?

Philip from Kamen Rider W
Ryoutarou from Kamen Rider Den-O
Kouta from Kamen Rider Gaim

Best Final Form (aesthetics-wise)

Philip.

That depends. Because mine is OOO, but which are we counting as his Final Form? PuTuTyra or TaJaDor? Because both are beautiful works of art.