Hi and welcome! Are you talking neck or bridge here? Or combos? Makes quite the difference.
For the bridge, the HD might be the best one indeed, but for the neck the field is much wider. Neck PUs to look at for amazing shred action: VHII, Aftermath, Emerald, Coldsweat, Painkiller, Blackhawk, Rebel Yell, Holydiver, Juggernaut and the Miracle Man. Plus I personally would not dismiss the Stockholm HSP90 (fluid, full, great attack, can scream and sing).
Then again, I think one can also shred one the Mule very well and I have heard the Manhattan sound kinda great there too. Thats the beauty of BKPs for you...
The most common and well tested shred PUs out of those perhaps are the Emerald and Coldsweat, but all can do it and with their own strengths. The Aftermath for example is quite lovely, I very much enjoy it especially for older Petrucci sounds, while the Coldsweat is more assosiated with his newer sounds. The VHII, if I recall correctly, has a bit of a single coil quality and is also very fluid, plus a bit more 80s in voicing. The HD neck is on the brighter front, while the Blackhawk is darker. The MM has a comperaticly Hifi sound (great for a lot of shred sounds) and the painkiller as it is ceramic is pretty smooth and unique too.
So you see, as far as shred necks go you really got choice. Thus youŽll have to tell us a little more about what you are looking for here ;)