Well most of the time the armor from the Kamen Rider Series is 2 Different Kinds... Depending on the scence.
#1 - The Armor is FiberGlass and Resin and then they will sculpt the front half and then the back half and connect the 2 with helmet clasps.. example picture here
of course this picture is to show you what it was made of. The Sculpted it out of clay first then molded and casted. It can be casted as one piece or as described front and then half. Either way you will need to leave yourself room to get in and out of it hence the clasps.
#2 - Almost the same as the first step sculpting it out of clay and molding it. Then instead of Fiberglass and resin you will use Foam expanding foam our some sort of pouring foam. Which you then will cover with a fabric type material which the name elludes me at the moment. You will of course for this method need to cut a a line down it in the back and add Velcro or a Zipper of some sorts to get in and out of it.
#3 - You can work out the deisgn of the armor in piece and buy a material called LionBoard which is usually found in Japan but there are some websites that sell the material. Then Piece it together till you get a Blackish Grey Version in which to finallize the detail you will cover it in a fabric material like white or what ever rider your doing and incase I forgot to mention you will be using fabric cement or some kind of adhesive to attach the fabric to the foam.
The Main Key with all this is patience... if you dont have it ..will turn out bad..lol
Hope this helps... I didnt list materials as there are too many that could be used... and different clays you can do and there is no best or I recommend I use too many myself to have one..lol