I recognize I'm being baited a bit here, but I think the current balance between top decks in various factions is pretty good -- it is matchup dependent, but not in a way that clearly favors one faction over another. Viable deck variety may be another issue, but that is a bit hard to precisely define. So rather than ranking faction by power, I will rank them by the amount I enjoy them. Of course, this is totally subjective -- I would not expect any level of agreement.
Let me start with my preferred factions:
1. SK: Maybe it stems from the nostalgia of having Olaf as the first gold card I found (other than Rivia which is automatic); but I enjoy selfwound. I love Artis for his ability to turn the game sideways without being OP. Knut opens so many tactical options. But I also enjoy a druids deck I'm currently playing. I like the healers and Svalblod (although it is inconsistent). And I enjoy graveyard play giving second chances. And I think SK's resilience to heavy removal helps keep the game strategically interesting -- responses are meaningless if nothing remains on the board to respond to. For that reason, I do not like warriors, but it is easy enough not to play them.
2. MO: In some ways, the simplicity of many monsters cards allows a bigger focus on strategy as the tactics are pretty self evident. It was playing monsters that most helped my skill development as a player. I enjoy the variety. Deathwish admits significant creativity. Thrive generates huge numbers of points, but very small decisions can have significant consequences. Vampires, especially devotion vampires are a lot of fun with significant surprises. And again, I appreciate a meaningful graveyard.
3. NG: More than any other faction, NG keeps extremes in check. Because most NG decks do not have powerful native cards, the faction tends to be highly interactive and strategic. It rarely wins on its own; it wins by preventing an opponent from winning. And this usually requires a lot of thought -- on both sides of the match.
4. Neutral: OK, it's not a faction. And there are a lot of neutral cards I really hate (even if I admit that they are necessities in the game's current state): Heatwave and Oneiromancy are at the top of the list. But if there were les disparity between top and bottom cards, neither Oneiromancy nor Heatwave would appeal. And I have not found a truly competative, all neutral deck. But neutral cards add consistency across factions, add a certain constancy to the game, and even help highlight how factions differ by providing different benefits to different decks. Especially notworthy in this regard are cards like the new Troubadour and Syanna. Many neutral cards add interesting features to decks without changing the faction based appeal of the deck. Even cards like Renfri and Golden Nekker, which remain horribly OP, feel very different when played in different factions. And several of my favorite cards in the game are neutral: plain Ciri, Vellentretenmerth, Mushy Truffle, Troll Porter, Wagon, Scepter of Storms, Squirrel, Wagenburg. I may not find them competitive, but I enjoy the strategy they introduce.
I now move to factions that I generally don't enjoy:
5. NR: Absurd engine value tends to make this faction extremely binary and predictable. The cards might change, but in large part, the strategy doesn't: build 6+ points per turn engine value to force an opponent out of round 1. In long round three, again play for 6+ points of engine value and trust that nothing reasonable keeps pace. Against greedy opponents, forget round three and exploit first say to set up engines faster than the opponent can.
6. ST: Should be retitled Simlas/Saskia because that's all you ever see. By far the most uninteractive faction in the game (and arguably the most binary/RNG), I find ST neither fun to play nor to oppose.
7. SY: I simply don't like the coin mechanism. Balance is incredibly fragile. The earning/spending mechanism is a deckbuilding nightmare. It introduces huge draw-based RNG. And it eliminates round based strategy because SY almost always needs either leader or carryover (usually coins) to have viable options.
