Power Ballad: I don't know if I have told you this - but there are a few songs of his which I feel he himself cannot emulate. They are: Tadap Tadap, Alvida, Dil Ibaadat Rock, O' Mama and Power Ballad (and Cineraria to quite an extent).
Some things can be done just once. The sentiment attached with these is amazingly special. And when I say KK himself can't repeat something he's done, that eliminates the possibility of any other artiste even trying.
Dil Ibaadat: Dil Ibaadat is effortless (not that most of his songs aren't), but this one's that kind of effortless, the kind of effortless magicians portray they are.
One of my friends said to me the other day, "Mera ek dost hai, Delhi ka...wahaan log KK ko baaki shehron se better jaante hain. He's an aspiring singer, but he says KK ke songs nahin try karta main, nahin hota mujhse."
I've seen people try O Meri Jaan (Metro), Alvida, Tadap Tadap, Ajab Si, Aashaaein and even Zara Si, on reality shows. And each time after they finished, I've had to listen to the original to distill the effect they had on my psyche.
Aasraa: It's difficult to describe this one. More so, because, firstly, I'm by no means a writer and secondly, I would have done better if I had been through the agony this song describes.
It's here that he freakin' outdoes everybody - he can emote like he's been through every-bloody-thing a human being can ever come across. It's pure supernatural-ism (if that's a word).

"...rehne ko tere baa-aheeinn mile..."
And, "...jaa-naa hai kahaan..." XD