While listening to Pudhupettai, I realized that it had an unusual number of 'music only' bits which were so far referred to as 'Theme music' or 'Spirit of -----' (movie name) etc. I think the first movie to have a separate theme bit was Punnagai Mannan, in which Ilayaraja had used not one but two bits.. AR Rahman, probably thanks to his association with Ilayaraja during Punnagai Mannan, realized its novelty and potential and started applying the concept for his movies more.. Here are my favorite theme musical scores:
1) Bombay: The flute bit is the best I have heard so far.. though the song 'MalarOdu malar indru..' comes much later, the music until then is mesmerizing...
2) Punnagai mannan: The part Ilayaraja had used from 'Nothing but wind' was very famous and sounded 'different' those times.. First time computer-generated music in Tamil cinema..(or Indian cinema itself?)
3) Rangeela: It was called the 'Spirit of Rangeela'.. What a beat it was.. RG Varma had to use 'slow motion' in the movie to rescue his choreographer.
4) 7G Rainbow Colony: Though Yuvan seemed to have been inspired from Spanish music, the score had enough variations and it was interesting..
5) Taal: Beat of passion.. starts slowly and gradually picks up momentum and ends up with a scream (unnecessary and lousy part though)..
The others I have liked are from Anniyan, Dil To Pagal Hai, Kadhal Kondenn and Pudhupettai.