Don't know if they're underrated as such, but certainly below radar in the audiophile community at large: the JBL 4306 and 4429 (the L100 could also be included here). They image well, are well-balanced, very dynamic, and just sport a clean, honest and low-colored sound.
What sets them apart from many of the more popular and typical hifi-speakers is the use of compression drivers in the mids and tweeter range, and to me - even though it makes them hybrid designs - this is a good thing. Horn profiles have come a long way these last 10-15 years, especially smaller horn variations (horn "honk" or other overt coloration in modern or larger, old horns is just gibberish and a flat out anachronism), the compression drivers have been great for even longer (though we've seen some improvements here as well), and all of this gets one substantially closer to having your cake and eat it too; there's refinement and imaging here to be had, in addition to significant gain in dynamic headroom, ease, sense of physicality/visceral feel, lower distortion and coloration (yes, you heard me right: lower coloration compared to dome tweeters and coned direct radiator mids). The more I get used to listening to quality horns the more coned direct radiators sound like cones. Piano, violin and saxophone in particular I find to be revealing instruments here, and when you go all-horn wait and see how bigger string instruments like cello and double bass can sound like.
Most overrated speakers? Not that they seem excessively popular, but I've never bought into the sound of German Physiks speakers.
