My two favorite mutes are both made by Denis Wick. Their straight mute has a good tone and holds up well to loud playing. It does not affect the intonation too much at all and has a decent response. Their practice was a great help when I was on tour and had to practice in a hotel room at odd hours. It quiets the horn nicely and plays the whole range without trouble. I had tried a H/B practice mute, but it really messed up intonation and response in the high and low ranges.
[[ UPDATED LINKS - THE MUTES ARE AVAILABLE BELOW ]]
Denis Wick Euphonium Straight Mute - Aluminum
Denis Wick DW5512 Euphonium Practice Mute