"Sol Failing"? That's a second "l" there, not an "i".
As for making the conclusion that Lelouch loved Shirley the most, I wouldn't go that far in the series. As a matter of Lelouch's canonical romantic experiences, it was pretty much stated that he was too busy with his rebellion to spare time/energy for romance. I find it plausible that Lelouch, as an empathetic character, would have been
willing to love any one of his major love interests (Shirley, C.C., Kallen) in the absence of extenuating circumstances.
However, the area where Shirley had an advantage over Kallen and C.C. is that where Lelouch lead/inspired/protected Kallen, and saved/redeemed C.C., Shirley was the person who saved/taught/inspired/corrected
him. Kallen and C.C. were enablers who supported Lelouch along his path of carnage. Shirley was the guide who gradually taught Lelouch that there was more to the world than hatred and destruction. Ultimately, she was a representation and protector of Lelouch's "Hope". After Lelouch's whole revenge issue with Charles was completed and crumbled, that was all that he had left.
It's not certain that Lelouch couldn't have been moved by any of the girls' love for him. That's the whole guy thing, "don't let a woman cry", etc. But Shirley had a way of moving Lelouch's heart,
even in matters unrelated to romance. She brought him a new perspective. That's why her love for him played such a pivotal role. It's also why, if Lelouch actually
did have a chance to live the peaceful life he dreamed of, it'd clearly be most fitting for Shirley to be by his side.