I think Mikael Akerfeldt is the best growler, but that's more of a matter of opinion than "technical ability" I think...it's rather hard to determine one's skill at making hideous noises. I just think his are the coolest sounding of anyone's
I personally don't think his clean ones are that great, i don't care if he's in tune or whatever, I just think his voice is kind of weak and that his clean vocals usually sound "flat"-they don't seem particularly emotional to me. Entirely a matter of opinion so don't try and start an argument.
As for guitar playing, Akerfeldt obviously isn't one of the top ones in term of skills. He doesn't even know the theory behind what he's doing, which while it doesn't mean what he plays sucks, means that he is automatically at a huge disadvantage to everyone who knows what they're doing cause he isn't able to easily improvise coherent solos, etc. Anyone who argues that "omfg this just means they are boring and HE ISN'T AND HE HAS FEELING!" is missing the point, improvisational skill and knowing what you're doing is part of musicianship, like it or not. This said I really do like his solos, especially on GR and MAYH (Ghost of Perdition's last one is probably my favorite).
In addition I'm 3000% sure he (or Peter) couldn't play most of the material from any number of technical metal bands. I'd like to see them attempt to play some of the nastier material by Necrophagist or Theory in Practice or Spiral Architect or even Dream Theater or Into Eternity, I'm completely sure they couldn't. I don't really LIKE most of those bands, but the fact remains that they're a LOT more musically adept. I can actually play 99% of Opeth's material (about the only thing I can think of offhand that I can't are the last solos from White Cluster), yet I can't come close to playing tons of material by Dream Theater, Pain of Salvation, etcetcetcetc. Opeth is really just not that musically impressive in the guitar department.
I can't really judge who's the most talented guitarist out there, there are way too many out there who can all play so fast it hardly matters who's more talented. John Petrucci has got to be up there, though. As far as FAVORITES go, I don't really have any, sry.
Bassist - He's pretty good, especially on Ghost Reveries, but it's just not even worth comparing him to the better bassists in the metal world. Guys like the player for Stargazer or the guy from Spiral Architect could play him under the table with one finger. Truly insane bassists. My favorite bassists are Michael Manring and Victor Wooten, who are way better than the two guys I mentioned even, but they aren't metal by any stretch of the imagination so whatever. Some other really good rock bassists are Stu Hamm, Billly Sheehan, and even Sean Malone (from Cynic, Gordian Knot, OSI, etcetc), the guys in Atheist, John Myung, and Steve Digiorgio (I forget how to spell that, guy from a couple Death albums). I will never understand how people can say Les Claypool comes close to almost any of these guys, especially Manring/Wooten.
Drummer - I am not the best judge of drumming, cause I can't play them for shit. Based on my untrained ear, Lopez is really good, though. He's not my personal favorite metal drummer (that would go to Giuseppe from Novembre, who has the most interesting drumming I've ever heard on a metal album on all of Novembre's albums, especially Novembrine Waltz and Dreams D'Azur), and I dunno if he could pull off the double bassage of a lot of guys (Giuseppe, Hellhammer, Pete Sandoval from Morbid Angel, etc), and I'm completely sure he could not play any of the material from any recent Meshuggah albums (especially Chaosphere or I, assuming I didn't use a drum computer like they had on Catch 33), but arguments I've seen about "feeling" and "latin touches" from drummers I can't comment on so maybe he is better than he sounds to this non-drummer.
And what is with all the Danny Carey worship? I have NEVER noticed anything impressive in the Tool catalog, but maybe I'm deafened by their dreary powerchord drop-d sludge of boring music to this apparently spectacular drumming. I sort of liked his stuff on Lateralus but I don't remember any of it being technically impressive.
Keyboardist, HA. There is absolutely no comparing the Opeth guy to the keyboardist in practically any band that uses a keyboard player as more than atmosphere. The best keyboard player in the rock/metal universe is probably Jordan Rudess. Whether you hate Dream Theater or not, there's no denying that guy's talent. Everyone in Dream Theater will even say that he's by far the most talented at his respective instrument of any of them.