So, there is this really common idea in fandom that Lelouch is more invested in Suzaku than vice-versa.

I do not agree, really. I think that Lelouch is the Point of View character, so we hear more of it from him, but that they are parallels in this way... and that it becomes fairly obvious, if you look at Suzaku's actions throughout the series.

This is not a nicely-formatted essay, be warned. It is really just a list of 'evidence'!

( And it is rambling. )
Ultimately, I feel like Lelouch thinks he's much less important to Suzaku than vice versa, and fandom believes it. But I don't think it's actually true.