I think Peter Hammill is one of the most impassioned prog singers- check out anything from 'Over', especially songs like 'Autumn' (the Peel Sessions version is even better) and 'This Side Of The Looking Glass', where his delivery drips with pathos.
Peter Gabriel's singing on 'The Musical Box' and 'In The Cage' is very emotionally rendered too, and Phil Collins (much as I admire him as a singer) didn't sing them quite as well.
But Phil Collins, a man so often criticised, has a first class voice in my opinion, and is arguably technically a better singer than Gabriel- just check out songs like 'Ripples', 'Snowbound', 'Undertow' and his brilliant version of 'Supper's Ready' (I actually heard the Collins one first, so I'll always prefer it...)
David Byron is a favourite of mine- 'Come Away Melinda' (which is possibly shared with Ken Hensley- they often sound very alike to me) is performed wonderfully, as is the lovely 'Wonderworld'. Talking of Ken Hensley, I always really liked his voice- it's not technically brilliant, but his delivery is hard to top on Heep classics like 'Firefly'.
John Lawton's singing on another Heep classic, 'Come Back To Me', is perfect for the song, but Bernie Shaw's version on 'Acoustically Driven' is even more emotional and impassioned.
Andy Latimer is yet another non technically perfect singer, but 'Rainbow's End' is a beautiful song and has a very sweet delivery by Latimer. I also like his singing on 'Who We Are'.
Eric Woolfson of the Alan Parsons Project's singing on 'Time' is first class, and tugs at even the most hardened heart strings...
Fish's singing on 'Heart Of Lothian' and 'Kayleigh' is utterly brilliant too.
Jon Anderson's vocals are totally inimitable- possibly because nobody else knows what they are about. Whatever, he's a great singer- songs like 'Your Move', 'The Revealing Science Of God' and 'Awaken' are brilliantly performed.