1. Q
2. Organians
3. Douwd
Where do you think Nagilum's people would fit on this list? Those were not trivial things it did with the Enterprise and surrounding space. At least not trivial to a Fed-level tech. What it did was equivalent in scale to what Q did with that giant curtain/shield in Encounter at Farpoint. I am not saying Nagilum is Q level, but what we saw it do could as easily have been demonstrations of Q power.
Which never happened.
Right, Yesterday's Enterprise is a might-have-been. But so is any wargaming scenario/projection made as a matter of course in a military organization, and these are taken into account when predicting the outcome of a military encounter. Yesterday's Enterprise is considerably better than a wargaming scenario, as it is more than a thought experiment--it really happens in one timeline, at least for a "while," if that has any meaning with time travel.
It both bugs me and pleases me that the Feds were losing in this ep. It bugs me that it takes me out of my comfort zone and forces me to contemplate that the most enlightened AQ power, the one I sympathize most with, is not necessarily the strongest, and it pleases me for the same reason.
I The Travelers convert thought to reality, so they may even should be just below the Metrons, but they used the Federation and others to get around, so they're not limitless in their expression of those powers.
Journey's End shows us that they don't need a starship to get around. He was on the Enterprise because Wesley was on the Enterprise. Also I think he makes statements that he might be from another time from the Enterprise's point of view. It is clear from the Traveler's statements that we are talking about more than gifts for spatial travel here. With time travel all kinds of bets on power are off. Although if this Tau Ceti native is any indication they are far from warlike.
My take on the Travelers is that they are not all Tau Ceti natives but are themselves a loose "federation"--really a special scientist club--of individuals like the Traveler and Wesley. They probably span many races, many times, and many viewpoints. They're basically us--fans--with powers. Although I'd also bet most are actual scientists.
But they might be very dangerous if they decided to really act in a concerted manner. I imagine them being very slow to take any action, they are free thinkers and an anarchic bunch probably, but when they do they sure have a dangerous brains trust and capabilities to draw on. I see a Travelers "council"--probably more like a scientific conference or symposium, hah maybe on Risa (but that is a little too high profile)--where talks like "cross-quantum entanglement integration across a manifold to erase unwanted Borg branching" or "imposing limitation on Federation expansion through time-juncture wormhole combining--an idea whose time has come?" are common, but maybe sometimes a consensus is reached and they DO something BIG. But they might always be meddling some.
Bit like Marvel's Watchers, less powerful (but still damn dangerous) but alike in temperament. Or Pratchett's History Monks (No Such Order, the Men in Saffron...). Or like...
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...How about Annorax? I guess technically the Krenim, at least for "a while," were unimaginably powerful.
Oh, maybe Annorax was a Traveler! A rogue Traveler...