Who are some of the best vocalists in prog?

As far as range and technical prowess go:

Peter Hammill

Steve Walsh (in his younger days)

Dan Swano

Fish (in his younger days)

Ross Jennings (lesser known, he's in a band called Haken and has an incredibly diverse range)

Then theres the singers who aren't as technically gifted but can still belt out some tunes with some real intensity:

Fish (in more recent years)

Steve Hogarth

Steve Walsh (more recently)'

Hasse Froberg

Roine Stolt

Nad Sylvan

Rob Sowden

Peter Nichols

Chuck Schuldiner

In many ways I actually prefer the latter singers to the former.

Glad somebody said it 

How can a screamer be technically the best?

While I agree that Bruce is not technically the best, there's much more to his singing than just screaming.  He used to sing at the top of a tenor's range all the time in the 80s so that is very demanding. Sure, we have so very many of those metal sirens by now but it's still not for everyone. And not just the fact that he hits those notes, he belts them powerfully. And he sings in tune all the time, so it's not screaming in the Tom Araya-sense at all.  Lacked adaptability and favoured an overwrought style of emoting but that's a different story.  

Well, maybe my objections come across as semantic, but Bruce is simply belting hard at high pitch. It's not shouting because he's very much in tune.  The last lines of Hallowed Be Thy Name, for example, are SUNG.  With a lot of power and fury, yes, but it's still singing. I would not call it shouting or screaming because trying to shout out those notes like that day in day out in concert can wreck the singer's voice. Bruce was absolutely in control and knew what he was doing.

Fine, but I raised the objection only because you are questioning another's assessment of Bruce's technical abilities based on what appear to be somewhat inaccurate observations.  You are free to call him what you like, but factually, Bruce is a singer, he's not a typical thrash metal shouter sort at all.