Now that all 3 have solo albums (technically Hikaru had some but they were strictly limited releases), I can compare their works so far, since it's also been, what, 4 years since Kalafina's disbandment.
Hikaru has some my favourite songs among the 3 (granted I'm a big fan of anison so that's not a surprise), but on a whole her discography is dragged down by the same-y boring indie songs she releases on YouTube, and her vocal technique. Her first album is a solid 7/10 but it could have been a lot better with more album-original songs instead of recycling her singles and B-sides. While she's singing in a much lower tessitura than during Kalafina, she can't control her voice with dynamics (it's either a terrible soft voice or a terrible loud voice) and her lack of range prevents her from singing better songs since there are only so many melodies you can make with one octave. I like her vocal performances in Avaricia, Ambient Border, Alternate (acoustic) and Gira gira especially, but the rest is a big miss. She either needs a complete vocal reset, or just get surgery for her nodules. Some great songs, most of them being decidedly meh, vocals need a lot of work.
Wakana is overall more consistent. Lots of ballads and with the occasional upbeat song. I like both her albums (Ruriiro no Sora and Ai no Hana from her first album are 10/10 songs, with where and magic moment in her second album take the cake). Even the acoustic album is fantastic (Rain is gorgeous). Her voice has certainly gotten a lot stronger (that belt during the magic moment live actually gave me goosebumps), but her head voice, oy it has declined so much. She doesn't even use it, opting for her much weaker falsetto instead. She just needs to not place her voice so high in her nose, changing vocal placement is singing 101, so I don't get why she still chooses to sing so honkily, on top of losing her vibrato completely. She does this annoying thing at the end of her lines where she drops the note - once you notice it, you can't un-hear it. Overall, her songs are good, vocals are so-so, it could be a lot worse.
Keiko...it's really sad looking at her solo career. While it didn't sell as badly as Hikaru's (I blame that on a complete lack of promotion), Keiko showed the most promise with a solo career since she had it all - the looks, the vocal chops, the stage presence. It's really sad that 95% of her songs are downright bad, either from terrible vocal choices (Hajimari wa and Ray) or just being bad songs themselves (Be Yourself is a walking disaster, every one of her rock songs are some of the worst I've ever heard). The other 5% are great (Tameiki, Tooriame, lost - that's it). She suffers from an identity crisis, since she has no direction or genre she wants to pursue. She has the technique to be amazing in all of them, we've seen that from Kalafina and FJ, but her composers are, for lack of a better word, incompetent. I've read somewhere this is what Keiko likes to listen to, but she can't be serious - her discography is a far cry from the Kpop and Jpop she listens to (she has said she likes Twice and SNSD, but their songs are FAR better despite being mainstream). None of Keiko's songs would even be considered mainstream since they're neither catchy nor listenable. If I had a choice, I'd want Kajiura to be her composer, but not in the vein of Nanairo no Finale, that was bad. Kajiura's recent singles from Lisa, Aimer and Hinano have been so good, like, 2007 Kalafina good. If Keiko sang those kinds of songs instead of...whatever she's doing, she'd be way more successful.
So, overall, Wakana > Hikaru >>>>>>>>>>> Keiko
But vocally Keiko >>> Wakana >>>>>>>>> Hikaru.