In the soundtrack of Guna, Ilaiyaraaja refers to a song which, according to him, Kamal had sung better than SPB! Which song was it, probably?

The absolutely fantastic song "Megam Kottattum" in Enakkul Oruvan.
While the SPB version is what is widely known and immensely popular, the version sung by Kamal Haasan is available at Raaga.

In the Guna soundtrack, there were some conversations between Kamal Haasan, Santhana Bharathi and Ilaiyaraaja, supposed to have been recorded by Gangai Amaran, along with the songs from the film.
Raaja says in one of those conversations, "Neenga paadneenga paarunga enakku, ennoda music'la, namma Kavithaalayam padathula, andha alavukku Balu kooda paadala!"
Kamal goes, "Ayyuyyo!"