I'd say the problem wasn't his attitude. I can like a character that feels like he's the best ever, if the series has him develop over it, or take it in a comedic way. At first they seemed to take the latter approach, having him be EXTREMELY over the top, which made him more hilarious than the Taros. The problem is that he had all the right to think of himself as super ultimate God, which made him go into annoying. Tendou is a gigantic God Mode Sue. He could do absolutely everything perfectly. Even Godai and Tsukasa didn't get to his level.
So we had a character that both had an annoying personality and was perfect. What else to add? Oh yeah! Everyone at first reacts as they should (seeing him as a prick), but he manages to get Kagami's friendship, despite being a constant ass towards him. There's no good reasoning for it, other than Kagami being an abused wife for Tendou. It's sad since Kagami is really the best thing of the series, and Kabuto would've been a far better series if it was Kamen Rider Gatack.