When I think which of the nations are strongest in each discipline, I don't think of the upcoming Worlds (I didn't that that was the OP's intention). Also I consider a mix of "height" (which strong medal contenders a country has) and depth (how many strong skaters do they have in the discipline, what do their upcoming Juniors look like). Based on that, here are my thoughts:
Men - Japan > U.S. = Russia > China > Canada
Ladies - Russia > U.S. > Japan > Canada > China
Pairs - Russia = China > Canada > U.S. > Japan
Ice Dance - U.S. > Canada > Russia > China > Japan
For the Men, picking Japan first was pretty easy. The hard part for me was deciding between U.S. and Russia. Russia has more proven, experienced Senior-level competitors in Voronov and Kovtun (proven being a relative term here, of course), but Farris in particular is coming on strong, and Brown has a great deal of potential. And with Aaron and Rippon (more wild cards but still lots of potential), as well as Nathan Chen coming along, I think there's more depth than with Russia with Pitkeev and possibly Gachinski if he could ever get it together. I've always found Menshov intriguing and entertaining, and it looked for a little while there like he might be a late bloomer of sorts, but lately his results haven't been encouraging, and he's several years older than Voronov.
After Russia in Ladies, I think both the U.S. and Japan can make a strong case for #2. But with Japan's loss of Mao (assuming she doesn't return to amateur competition) and Akiko, and Kanako not performing well lately, I don't think Miyahara and Hongo are enough to put them above Wagner and Gold, even with all their question marks. And U.S. has Edmunds, Chen, possibly Nagasu (hope springs eternal), Bell, etc. in the pipeline. Japan has some interesting Juniors, but I think the U.S. might edge them out in Ladies. For China, I think Li is stronger than the Canadian ladies, but with Daleman, Mallet, Chartrand, and possibly Seguin (fi she still competes in singles), Canada has more depth.
For Pairs, like others I think a strong argument can be made for Duhamel/Radford as the top pairs team, but after that with MT/M 1.0 gone, I don't think the newly formed partnerships are quite ready for prime time yet--although I think Iliushechkina/Moscovitch may well get there. And Seguin/Bilodeau also look promising. So with that in mind, I'd give the nod to Russia and China.
For Ice Dance, although Weaver/Poje are in strong contention for the top spot at Worlds this year, I don't think they're that far above Papadakis/Cizeron and Chock/Bates. And even if they were, the U.S. also has the Shib sibs and Hubbell/Donahue, both of which I think are stronger than Gilles/Poirier, and I think Hawayek/Baker are stronger than Paul/Islam and have some wonderful potential. Also Cannuscio/McManus looked good at Nationals.
I feel more hopeful about the U.S. than I have in recent years, and on paper it seems like things could go a little better. But the proof is in the performances, of course. And these thoughts, of course, don't take into consideration skaters from smaller countries like Spain (Fernandez), France (Papadakis/Cizeron, James/Cipres), Kazahkstan (Ten), etc.
I'm a fan of skaters from everywhere and really hope for some strong performances at Worlds all around.