It's all very amp-dependant sometimes, IMO. And personally, I like some of the classic Celestion speakers which "colour" the sound significantly (eg, greenbacks and V30's).
For Marshall-type sounds, and definately with my current Plexi, the best speakers I have tried have, unfortunately, been the most expensive !
Scumback H75 and M75. They manage to have a sweet, pleasing treble response, without a hint of harshness, which I have not heard in any other speakers.