Which ever way it is, the most important thing is to share some knowledge. I mean, what do many think a step-by-step picture or video will get them? The chance to draw the exactly same thing? That won't help on the long term, and often not for other things. WHY things are done the way they are done is important, so this knowledge can be adjusted to other matters. So giving some understanding on the matter is what must be given through a tutorial. :3
While playing the same role than step-by-step deviations, videos are far more dense than long deviations and far easier to make. Of course the viewer has to stop at high frequence, but in my opinion it's worth it since you see not only screenshots, but also how the artist proceed (movements of the brush, tool adjustments, time taken, contruction of the whole image: all these little points that are a pain to describe in a written manner). So definitely video
I like the long, step-by-step tutorials people make. I personally find it easier than messing with starting and stopping a youtube video to try and follow, but that's just me. Any tutorial is helpful though, I love when people make them \ovo/
wish I could say any and all, honestly anything is helpful, and I find it easier from deviations cause I usually miss streams, and its good as a resource to just go quickly back to when I want as a favourite, but streams are great because you get to see it happening and can ask questions live... so really anything is great. Your speed paints are also cool cause they show colour palettes, rough ins for perspective, ideas in composition and more on a variety of pictures.
Speedpaints are actually really really helpful; In order to learn I just need to see the brush and in what order and manner of brush/pencil/pen strokes it's drawn in. That's me anyway, I learn by mimicry and experimentation, but others might have their own styles.