The title is illustrative, but it can be added that there are no comparable alternatives for it, at least at the time. Moreover, it is predicted that finally some alternative will be presented, however, there is no info about its quality or details.
I don't think I can come up with something that would encompass both - that this is by far the best alternative and that whether it will remain so in the future is uncertain. As it seem logical that the first part is essential I suggest:
Not able to be exceeded in quality or degree:
For those who don't know, he was the unsurpassable genius responsible for the first Incredible Hulk and Fantastic Four strips - in other words, one of the greatest visual artists of the twentieth century.
For the uncertain future you can use something like:
Until now or until the point in time under discussion:
As someone who has hitherto needed to reject one thing before moving on to another, Juliet is on her mettle, and she knows it.
Although, you should be careful with this and similar expressions (so far, until now etc.) because it might depend on the context whether it will mean that until now it was so, but you don't know how things will be in the future, or that until now it was so, but for some reason it isn't any more.
The Latin (now English) phrase sine qua non means
An essential condition; a thing that is absolutely necessary: grammar and usage are the sine qua non of language teaching and learning
It literally means without which not.
A phrase that connotes high quality is state of the art
The most recent stage in the development of a product, incorporating the newest ideas and the most up-to-date features.
However, this term does not require that the product or service is unique. A number of examples might be state of the art.
The term nonpareil does convey unequalled
Having no match or equal; unrivaled: he is a nonpareil storyteller [POSTPOSITIVE]: a film critic nonpareil
all definitions from Oxford Dictionaries Online
I'll give it a shot:
The guitar skills of Jimi Hendrix are incomparable
The statistics of Sir Don Bradman are unparalleled
The success of the iPhone is unprecedented
The career achievments of Meryl Streep are unrivalled
The beauty of Miranda Kerr is matchless
And the most fancy of all...
- Da Vinci was a nonpareil artist. (can also be a noun) (Oxford)
My suggestion is quintessential:
Representing the most perfect or typical example of a quality or class:
he was the quintessential tough guy—strong, silent, and self-contained
An idiomatic phrase that is used to refer to someone departing that was held in high esteem is: a hard/tough act to follow.
to be so good it is not likely that anyone or anything else that comes after will be as good
Last year's thrilling Super Bowl, when the New York Giants beat the Buffalo Bills 20-19 will be a hard act to follow.
The new Chairman knows his predecessor is a tough act to follow.
The Free Dictionary by Farlex