Actually, now that I think about it, if you include the educational models with the buttons in place of keys, and cassottos, Hohner made five types 26s and five types of 27s. (The cassottos are not called “pianos” but look just like the HM Piano 26s and Piano 27s except for the chamber on the back).
So which models are best? I just purchased one of the metal Hohner Piano 26s with the small keys, and a photo of it is included below. As Melodica-Me and Andre stated, these sound wonderful. What a great smooth tone. This is the way I like my melodicas to sound. They may look like kid stuff, but the design, build quality, and materials of construction are perhaps the best of all Hohners except for the high end 36s. When you pick this guy up up, it is like picking up a log. It is very compact, something that I don’t mind. It sounds smoother than my cassottos.
On the down side, it lacks some volume and the keys are slightly narrower than most Hohners by about 1/2 inch total over two octaves. That is probably not really a negative, because I hardly notice it. The stubbiness of the keys does not seem to be a problem either, at least for me. The biggest problem may be perceptual. The audience may think you are playing a toy. I am going to play it in public anyway. I love the way it sounds.