As with most things, that depends on your requirements (bands, efficiency, power handling, …).
I started my experiments with the well written document of Heinz @HB9BCB (in German) and continued my experiments that resulted in the following document:
At that time, I was not aware that Heinz experimented as well with the same toroid core. My main goal was to design a 20m long portable multiband EFHW that is resonant on more bands than the usual 40-,20-,15- and 10m bands and requires a minimum intervention to change the band segments. The solution was to insert one coil with one bypass link or switch.
The majority of transformer designs that one finds on the web and even many commercial products are not very efficient. Just read Heinz’ articles in this reflector and you’ll find out.
They work, but Owen Duffy would say: “Well, in ham radio, everything works. But systems that work better increase the prospects of contacts.”
73 Stephan