wow, this vote is too difficult, in my opinion.
If you just focus on the growls, I would say that Johan Hegg and Åkerfeldt are almost equally good. After them come Mikael, Mr. Swanö and Tompa. I love Mikael's vocals on Haven - on that record he's the best
Gothenburg death metal vocalist - I can't imagine Åkerfeldt to sing for a band like At The Gates or Dark Tranquillity, his vocalst just wouldn't fit the style, because it is faster. There would be only very few passages where he could use his very "long" growls. Maybe you've heard his, Jonas Renske's and Roy Blackheim's project "Bloodbath" - the music is much faster and not epic at all, that's why I don't like his vocals on that one. The same goes for Johan Hegg (Amon Amarth _are_ fast, but they have these epic sounding passages where he can use his brutal growls)...besides that I also can't imagine him singing for DT.
About Johan Liiva: I like his vocals, but IMO they didn't fit Arch Enemy. At least not on their 2 last albums. I don't know why, but he just doesn't fit. I have to agree with Phyre that Angela sounds better than Johan when 'singing' for Arch Enemy...but I'm curious how he will sound on the NonExist debut album, I already heard some positive responses.
Anders Friden has/had potential - he was way better in the past: on the Jester Race and Whoracle albums, but nowadays his vocals suck (sorry, I just try to be honest
). I don't even like In Flames' music after Whoracle...
If you didn't focus only on the growls but also on the clean-
singing abilities, I must say that Dan the man Swanö rules supreme. After him comes Åkerfeldt and then Stanne.