The more recent the more exciting, also because most displays are interactive and you're literally allowed to touch and play with everything. Old wooden toys, an ancient canon with which you can shoot at the Burmese (the original enemy), a TV recording studio where you can read the news, which will afterwards be displayed on TV's all over the museum, a 50ties diner with an mini parked in the front, dress up in old-style Thai clothes and much more. A great place to take lots of fun photos.
Bad points are that the interactive games are often broken (I visit regularly), and the interactive displays and videos are sometimes very noisy.
Entrance fee for foreigners is 300 baht, but there's free entry on national holidays and during events. More photos • Museum Siam