The past three days, I have been listening to albums from my favorite prog metal bands: Dream Theater's Awake, SDOIT and DT12, Haken's Affinity and The Mountain, Icefish's Human Hardware, and Fates Warning's Theories of Flight. Something struck me.
The drummers cancel each other out. Portnoy, Mangini, Donati, Jarzombek and Hearne pretty much match each other. They all have distinct strengths, but liking one over the other is really about taste because they are all amazing drummers. Hearne is very underrated, he's like a bastard child of Portnoy and Mangini.
Keyboardists, Jordan is the virtuoso, but the Haken tandem of Tejeida and Henshall, and Icefish's Alex Argento can match him in terms of making beautiful soundscapes (and sometime they're even better in terms of creating moods and memorable melodies).
Bassists, I am really partial to Myung, but Conner Green and Joey Vera are amazing as well.
Vocals, I would say LaBrie at his peak really has the edge. But average LaBrie, the others can match him. Especially Ray Alder.
However, when it comes to guitarists, the difference is night and day. John Petrucci lords over them. Nearest to him is Marco Sfogli, but even then Marco just sounds like John's apprentice.
Which is why John Petrucci is Dream Theater's ace, which distinguishes them from the other big names in the prog metal scene.