After painstaking analysis of Led Locket Lising, I came up with this simple algorithm:
Find the number of correct English words found in the original lyrics compared to my transcription.
Find the number of words not found in the original lyrics compared to my transcription.

We got this:
Name: Found/NotFound

Riyu: 30/35
Noria: 17/24
Yoma: 10/14
Miharu: 26/15
Sayaka: 15/11
Risa: 10/8

In percentages, this is

Noria: 41.46%
Yoma: 41.67%
Riyu: 46.15%
Risa: 55.56%
Sayaka: 57.69%
Miharu: 63.41%

I'd say this is about accurate.
Miharu's certainly not perfect, but she did deliver her lines well enough.
Sayaka... well, BURAZING. But, they were still done alright.
Risa was pretty bad, actually.
Riyu was just... unintelligible.
Yoma... DON'T PEE ON THE RAINBOW!
Noria has the distinction of not only being completely unintelligible, but mispronouncing her part of the TAKAKU section. Listen to it. BLIGHLER.

So, my vote goes to Miharu. Discuss!

Noria's English in TEARS sounds okay, but only okay.

I'd have to hear them all sing a song right now in English. Riyu's English has been improving, and I don't know about the rest.

i think miharu too. idk why she just seems like she would know more english than the others. :]

I think either Riyu or Miharu.  Riyu's English makes me think a little because it can go from fairly well (Yamato Nadeshiko) to really bad (Baby's Tears).    Most of it in Rx3 is ok... I have to listen to it again to make a final decision....

For Riyu, it seems to depend on the speed she sings in.

  my vote goes to riyu,
most of her lines in the rx3 english edit are quite understandable

  miharu is like a mess hehehehe no offense
there was this line "In this tiny part, my mind" she sang like "in this tiny fabolay" hahaha

Noria.

From the side of the green bridge yeah!

Riyu

Go way, get away, my way!

I would have to say Miharu for Rx3 Riyu and noria were pretty bad in that song. Riyu is good in yamato and guilty sky though. it was a tough choice but I went with Miharu.