Diatonic harmonicas: these are the standard 10 hole harmonicas. Each is tuned to a specific key, this is why harmonica players have multiple instruments (more on this here). When I started, everyone used the Hohner Marine band harmonica. Many still do.
New harmonica models have since appeared, there is more choice. In the late 1990s I developed a diatonic harmonica tuning which I call “Major Cross”, devised for bluegrass and Irish fiddle tunes. Details of this tuning are here. In 2017 Seydel released a Major Cross harmonica, shown opposite. We worked together on this, needless to say I am very happy with the result. Thanks Seydel, I’m now a proud Seydel endorser!
In recent years I’ve taken to the tremolo, and like it very much. It sounds like an accordion, to me at least. I’ve created a set of online tremolo harmonica lessons, they are here. More details are at TremoloHarmonica.com.