I've owned a lot of inexpensive and relatively inexpensive gear in my day, but one eyepiece that jumps to my mind when I think of "underrated" is my 8mm Tele Vue Plössl. It is a much maligned eyepiece for its short eye relief (6 mm) but for me it is a wonderful eyepiece. I can acquire the exit pupil easily and see the whole 50° field. With my 8" SCT it provide about 250x which on most nights here is the max I can coax out of my SCT (although I've had it up over 500x on good nights!).
I know the TV Plössls have a good reputation - especially amongst planetary observers. So in this regard, I think where the 8mm is underrated is related to how easy it is to use. Compared to regular 6mm or 9mm Plössls I find the Tele Vue a step up in both quality and ease of use. Many people won't go near a Plössl eyepiece that is less than 15mm in focal length. And I know that tolerance for eye relief is a very personal thing - my tolerance is very high apparently.
Anyway, cheers to the TV 8mm Plössl and all the wonderful views it provides me.
Do you have an eyepiece (or eyepieces) that surprises you? It could be expensive or cheap (or in between) but just has a quality that was unexpected or performs in a way better than expected.
Clear skies!
Rick