Suitable word or phrase to replace 'unique' when something is almost unique?

Solution 1:

Maybe novel captures some of the meaning you're trying to convey? It suggests something out of the ordinary and interesting.

Verification of space weather forecasts presents novel challenges when compared with other types of forecasts.

Solution 2:

"Extraordinary", "uncommon", "incomparable". "Rare" might also work here.

Solution 3:

I would suggest that it presents some particular challenges:

particular
2. Separate and distinct from others of the same group, category, or nature: made an exception in this particular case.
3. Worthy of note; exceptional: a piano performance of particular depth and fluidity.