John,
Your question is one that can inspire lots of debate...for me the two at the top of the heap in terms of quality/performance are the Astronomiks (available at Bintel...I think they are on sale at the moment) and the NPB Filter from DGM Optics (sold by Omega Optical in the US and they will ship overseas)
http://www.omegafiltersebuyer.com/servlet/StoreFront
from their website***
NPB 1.25" – DGM Optics™ Nebula Astronomy Filter (price USD$75)
The NPB (Narrow Pass Band) filter is an Ultra High Contrast type of filter that is very effective for small and fainter emission and planetary nebula and a variety of bright nebula, especially through larger scopes
***
I have owned the Astronomiks and the DGM and for me when compared side by side I preferred the DGM (just barely)...On my scopes and to my eye it seemed to pull out just a hint more nebulosity...admittedly there was not much between them to my eyes.
I have not owned but I have looked on a few occassions through other brands of UHC filter and I'd stack the DGM Optics #1 and the Astronomiks a very close #2...then everything else
Although I have not tried one myself, I have read that the Orion Ultrablock (also avaialble at Bintel I think) is also a good performer.
I would likely rank other types of filters differently (OIII etc.) but for the "UHC" kind I'd place them that way...
Just my limited opinion...good luck!