I tend to agree with you that Rhavin probably is placed too high ... but RJ also made a point of showing that skill was more important than strength, look at Graendal, she's the second weakest Forsaken, but from the books was vastly more accomplished than those of much higher power!
And an another important point is that Graendal's fame came largely from her non OP skills. We knew she treated the minds the OP couldn't touch, and we don't know if the code of ethics of the AS let her use some forms of compulsion benevolently or if this was forbidden and she developed those skills, helped by her psychiatry background, only once a Forsaken.
Rhavin is largely described as a politician/diplomat. It's quite possible he never shone, despite his strength, as a channeler (the same can be said of Balthamel, for instance, and Moghedien). He surely was well trained, and surely he had to lend his Talents to tasks required by the Guild like everybody else, but it doesn't sound like he made a "OP"-focused career, the way Semirhage, Aginor, Lanfear or even Ishamael (as philosopher of the Wheel/Pattern/metaphysician) did. Demandred, like LTT, seems a bit of an unknown.. obviously both were very skilled channelers and it sounds likely they owed their ascension in the Guild to the merit of their service as AS, but as for the details we'll never know.
I do think Graendal should have had a higher strength rating given the comments from the books, or rather, that some of the other Forsaken should have been a bit lower as 3 doesn't seem too low for Graendal so much as Mesaana and Moghedien seem too high from the books. Semirhage was a little of a surprise, but not shocking. Mesaana as a 2 seems at least 2 levels too high and Moghedien seems like she should have been a 5, which also would have fit better with Nynaeve being her equal at one point, but still growing throughout the series without actually hitting her Level 3 potential. Similarly I would have thought the men would have been on at least 1 or even 2 more levels of the ++ ranks. From the books it seemed to me that the men should have been:
Ishamael, Aginor ++1
Demandred ++2
Sammael ++3
Balthamel ++3
Rhavin ++4
Be'lal ++4
Asmodean ++5
But to your point, Graendal clearly achieved her status through her career as a psychiatrist, despite her clear ability with the OP I find it strange that next to Moghedien she is the weakest of the Forsaken in strength.
Except that the books are not subtle at all regarding her prowess as a channeler. I have a strong feeling that she was strong in all five powers as well as unusually skilled with saidar, much like Egwene (I think Semirhage was too). She has no qualms taking on Sammael, Cyndane and Moghedien directly, and during the Last Battle she fought a group of the Light's most powerful channelers all at the same time. In many ways she was shown to be the most dangerous of all the Forsaken. I don't think it makes much of a difference if she was say 20% weaker than Ishamael if she has so much else going for her.
My male list would be...
Ishamael ++1
Demandred, Aginor, M'Hael ++2
Sammael, Rahvin ++3
Be'lal, Balthamel ++4
Asmodean ++5
Also remember that the Collapse took almost a century, and even the most unsuited channelers had plenty of time to learn to become lethal warriors with the Power. Peaceful non-channelers were eventually taking part in Sha-je duels... imagine what channelers were up to. After that they then had a full ten years during the War of Power to hone in on these skills, including a year where only balefire was used. Look at how dangerous Rand and Egwene became after only three years. If one reduced them to 'village people', it would be a dire mistake.
I personally found Graendal to be the most dangerous of the Forsaken by a long shot. Ishamael aside anyway ... but IMO he was something beyond the Forsaken and closer to a dark version of the Dragon and his use of the True Power really does set him apart. I like that her backstory showed us that even before the Collapse she was abrasive and obviously was a person who managed to make the most out of her world view. I suspect she innovated with Compulsion as part of her psychiatric work long before it was known. She's a great example of translating personal knowledge into functional OP ability.
Semirhage was a little of a disappointment to me as a character to be honest, she was set up so well to be a great villain, but I don't think she was well executed. She ended up a little flat IMO. Graendal started out very undefined, but became interesting as the series progressed, and her showing at the Last Battle was impressive, much more so than any of the Forsaken IMO.
Mesaana should have been weaker IMO ... Level 2 just doesn't match up all that well to the books to me. She was more like Moghedien to me, braver certainly, but still kept most of her agenda under deep wraps for fear of being caught out and she was outright dismissed by pretty much all of the others (even her allies didn't seem to think much of her).
Sammael could have been a better villain, but RJ was clearly going more for the action/adventure dueling situations with the early Forsaken (Be'lal and Rhavin weren't much different really).
Demandred was a let down, would have much prefered the Taimdred storyline I think. And ultimately he didn't get enough screen time in the series to be more than a "what's going to happen" situation.
I actually enjoyed Moghedien for the most part, in many ways she felt the most "human" of the Forsaken, her motivations were petty, but understandable and her cowardice was somewhat fun to observe, especially when she was convincing herself that her cowardice was actually just the smart play
Lanfear was pretty bland to me. When she first came on the scene I thought "okay, we have a dark temptress" ... then she just turned out to be a self-deluded psychopath. Wish RJ had given us more of Cyndane's story as I think seeing the mind-trapping from her perspective would have been much more interesting than what we got from Moghedien.
Asmodean I liked as a character, he was straight forward IMO and maybe could have been on his way to some sort of redemption if he hadn't been killed.
Balthamel/Aran'gar was a nice twist, and I found Halima's personality amusing and the interplay between her and the Aes Sedai was mostly well done.
Aginor/Osan'gar, Rhavin and Be'lal weren't really defined enough to care all that much about. Wish they had done more with Dashiva while he was with the Asha'man. RJ hinted at some of the manipulation he had been up to, but didn't show us enough to make it really pay off.