I go through phases where I find landscape easier, and then I just seem to "lose the feeling" so I focus on still life for a while. I have the same kind of relationship with my paints- some days I am in love with oils, on others I much prefer acrylics. Funny enough, though, I seem to use oils more for landscape, and prefer acrylics for still life. My guess is that's because I like to paint really bold, colourful still life compositions, so acrylics work better for that... while oils give me more subtle transitions of colour and allow me to make changes quickly, useful when working outdoors.
Landscape, to me, is quite connected to emotion- I tend to paint places that I love, and I want to convey the feeling of being there, not just the visual. Still Life, on the other hand, is more of a technical exercise- something I do because I enjoy challenging myself, and I can focus on a particular element- colour, composition, texture, reflection. And if I get it right, the end result is aesthetically pleasing enough for other people to enjoy it too!
Landscape is a big genre. I find woodland, hills and fields come fairly naturally to me, and I adore painting greens. Rivers and lakes are good, but coastal scenes are harder. Snowy mountains are more of a challenge again, and ugh, I can not paint deserts. Which makes sense, as I don't visit the beach often, live in a mountain-less area, and I avoid hot climates like the plague. If I'm painting en plein air then a lot depends on the lighting, temperature, and how many people come up to talk to me (and how social I'm feeling) on a good day, the picture paints itself. On a bad day, I spend hours fighting with it and get nowhere.
And still life- some days, I can't paint enough of them, I feel so enthusiastic and it just flows. Other days, I go through my collection of objects, raid the fridge and garden, and nothing inspires me in the slightest. I choose some items, spend an hour or two trying to arrange them into a composition, and eventually give up and go off to sulk for a while.
I think it varies a lot by artist, so I don't think there's really a "standard hierarchy of difficult"... And I think difficulty for me depends on the particular subject, rather than the genre. A patterned ceramic gives me more of a headache than a local landscape, but a foreign landscape will be harder for me than fruit.
Sorry for my massive long reply haha, I sit down to write a few sentences and end up with a novel... I hope something in my ramble interests/helps others!