Changers, but w/e floats your boat.
Best would have to be the Accellular and ShodoPhone, Why? Because They use their functions way more besides Changing. Like the Spinning sparker (name? lol) they use it multiple times for mecha, calling, and changing.
With the shodophone I can see (and so far they are) going to use the brush mode alot for new gattais, new kanji, and possibly power ups.
They use the "gimmick" as much as possible.
Best looking: Gong Changer, Shift Changer, Wing Trigger, Gorai (sp?) Changer, GoGo changer.