It's so hard to pick either. I hate to be long winded about my answer dear OP !
Wanderlust.
Biophilia.
It's probably the only B album I can pick apart, almost by every track. Now, not to say I hate it, but it could have been better imo. My take on it after all these years is that it was leaked early, and she was rushed to release what finished at the time to combat this. I don't think she ever intended to release the version she did.
That said
Issues: Some of the vocals weren't recorded with decent microphones or in a studio, and you can tell if you listen carefully. Her vocal performances of some of the songs aren't that great unfortunately and it makes some songs sound like poo.
Moon would be a good example of this. The vocal take itself sounds like it belongs on a demo, not a final version. To compound this further, the melodies of the choir, harp and the ambient sounds in the track are crystal clear and sound great, but when you add in her vocals it just sounds kinda unfishinished. It could have been better.
Thunderbolt is an amazing track, but the album version is a total demo. It just sounds so bad. The vocals here again could have been better, and the synth should have been replaced with actual "electric" sounds of some sort. After hearing Thunderbolt Live for the first time I was quite underwhlemed when I heard the synths on the album version =/ .
Crystalline - where do I start? I love the melody, gameleste and the beats. The added ending was an unexpected surprise too, however - this song makes no sense. Its worse than Earth Intruders as far as lyrics go, which is a shame. I get the idea behind the song and what it means as far as its scale on Biophilia, however as a pop song, it just falls flat completely because of the lyrics. I will however note a total contradictory thing in my review, one of her best lines ever is "its the sparkle you become, when you conquer anxiety". :P
Cosmogony - Very cute song, and good vocal performance overall, but the verse "And they say back then our universe was an endless land, Until our ancestors woke up and before they went back to sleep, They carved it up into the world we know" always throws the song off for me. The other verses all jive together well, but that one makes the song seem a bit off.
Dark Matter: LOVE this track and I LOVE to sing along
Hollow: Perfection in every way.
Sacrifice: I wish she had spent more time with this track. It's so pretty and I love the pin barrel harp. The melodies here are just outstanding, but the ending seems a mess? I completely love the lyrics here and her delivery. The ending WOULD be the best part about this track, but she chose to drown it out with beats. Now, I don't have a problem with the beats themselves, but they ruin the best lyrics of the song and also the woo woo sounds that she is making. It would have sounded MUCH much much better without the beats.
Mutual Core: Great song, but it is too short.
Solstice: The track is very lovely. I love the gravity harps that she uses here and how they blend together. Vocal performance was on point here, but I would have preferred a studio version. I didn't think the original version sounded all that bad truly. It was about the same vocal quality as Moon though.......I'm guessing she chose to go with the live version simply because of her delivery of the line " lightbear-er, receiving RADIANCE from others~s" was so much stronger...but still. A studio version would have been more proper for a B album.