Right, here I can have my say. And I be the undisputed expert and my words be the law ... ahem ...
Anyway:
Johan Langquist (Epicus): His performance on the album is as titanic and monolithic as the material itself. It is the essence of power, purity, beauty and perfection. Seeing/hearing him perform the Epicus material live with Candlemass twice has been a dream come true. However, his audition demos for Chapter VI and King of the Grey Islands shows in my opinion why he shouldn't be in the band permanently. If they would do an album together "musical differences" would probably be given a new meaning. Better not tarnish the legacy that is Epicus.
Messiah Marcolin (Nightfall, Ancient Dreams, Tales of Creation, Live, self titled): The classic voice of course. Tremendous vocal power and with a "I'll conquer the world and make kings bow before me" vocal style that'll lead you to believe his range is greater than it is. Some say he's just dead pan delivery and no emotion, I don't quite agree. "At the Gallows End", that's emotion for me.
Thomas Vikström (Chapter VI): He got so much unfair criticism it's ridiculous, especially in Sweden. He was labelled an "AOR singer" unsuitable for Candlemass and it all boiled down to that he was the singer of Talk of the Town that had a big hit back in 1988. In truth, he's an extremely versatile singer that was perfect for replacing Messiah. His performance on Chapter VI, despite the weak album production and the more up tempo oriented material, should speak for itself. "Where the Runes Still Speak" gives me goosebumps every time. Live bootlegs with the Chapter VI lineup is very rare but very precious to me. He could sing anything Messiah ever sung and then some (in fact he hit notes that were on the albums that Messiah never sung live, like the highest notes in Well of Souls). Some of his best numbers were Crystal Ball and Dark Are the Veils of Death. Here is a live version of Crystal Ball in 1993:
http://www.youtube.com/watch?v=ns8i88VnMHM
Björn Flodkvist (Dactylis Glomerata, From the 13th Sun): Surely can't compete with the others in terms of range or technique and is an entirely different deal, as are the albums he appears on. It was different and originally not meant to be Candlemass. His Ozzy like delivery was a great casting for From the 13th Sun, which is a fine album in its own right but I have a hard time thinking about it as Candlemass.
Robert Lowe (King of the Grey Islands, Death Magic Doom): Was a favorite singer of mine since long before he joined Candlemass. His performance on the first two Solitude Aeturnus albums is immortal. His voice and vocal style has changed quite a bit since then and he is very much like Dio these days but still with more feel than technique. I like hos vocals on every single album he ever sung on and as for Candlemass he really nailed it with Death Magic Doom. However he has severely let Candlemass down in the live situation by failing to learn lyrics and even when to enter verses and such. Everybody just to say Candlemass were better live than in the studio, nobody would say that anymore.
In conclusion, a ranking. This is only based on their performances on albums and only in Candlemass:
Marcolin > Langquist > Vikström > Lowe > Flodkvist