Wikipedia has basically a complete monster list, if you're curious about those.
As far as actor participations go:
Tv show
->The man who sent Tsukasa on his jorney is Wataru Kurenai, main character of Kamen Rider Kiva.
->Kick and Punch Hopper had their original actors voicing them in Episode 2 and 3.
->Kivat had his original voice actor in all participations. Kivat as part of a Diend Summon, Kivat with prince Wataru and also Kivat with Wataru Kurenai.
->Spider Fangire, who appeared in the beginning of episode 4, had his original actor.
->Kamen Rider Kaixa had his original actor voicing him in episodes 4 and 5.
->Shoichi Ashikawa's actor was previously Kamen Rider Kirameki in the Hibiki movie.
-> There were no replacements for any member of Den-O's cast, although they used the actor of Kid Ryotaro, from the first Den-O movie, rather than the adult one.
-> Ibuki, Zanki, Todoroki, Akira and Ouja in the Hibiki World episodes all had their original actors.
-> Otoya, Dark Kiva, also kept his original actor.
-> The 3 Riders in Diend World are based on the Riders from the Blade movie. They all kept their original actors too.
-> Black and RX had their original actors, of course.
-> Schwarian, the new Crisis commander in the RX World, was voiced by Tetsu Inada, the same person who voices Rider 1 in the game Ganbaride and in the Decade movie All Riders.
-> Kazuma Kenzaki, Kamen Rider Blade, who appeared in the final 2 episodes, had his original actor.
All Riders
-> Double, Kick Hopper and Ouja had their original actors voicing them.
-> General Jark had his original VA.
-> Agito's actor was the original, but he apparently didn't voice the in-suit scene due to schedule conflicts, only actually playing the character in the out of suit scene.
-> Every Rider that has had a VA officially listed for this movie, and wasn't voiced by their original actors, had their VA from Ganbaride (apparently even in cases where they hadn't appeared in Ganbaride at the time, like Amazon. The only exceptions happen when their original actors return to voice them in Ganbaride later, like apparently happened with ZX's).
-> Kenichi Suzumura voiced both X and GaraGaranda in this movie.
-> Tomokazu Seki voiced both Amazon and Ikadevil in this movie.
Movie Wars
-> The Blade that briefly appears in the first part of the movie is voiced by Kenzaki's original actor.
-> In the first part of the movie, every Rider besides Blade had their voice from Ganbaride, when applicable.
-> Because AU Faiz's actor didn't return, Kenji Akabane, the same replacement VA chosen for Skyrider(although he hasn't appeared in Ganbaride yet), voiced AU Faiz in the final battle (although he just grunted anyway).
