AR Rahman- A pure genius - Dude you rock !
"Fountainhead" is my all time favorite book so is "AR Rahman". If you have read fountainhead, you would have understood why Rahman is "Howard Roark" of music.
Every time I hear his mesmerizing compositions, I can not stop gasping about his genius. I believe he is truly gifted and his music is capable of making god's presence felt around you. Its a tall order to praise Rahman for his music because its literally means stating a universal truth.
I am always overwhelmed with his unparalleled ability to create new sounds and what intrigues me most is, he does that every single time.
In this world of mediocrity, doesn't he ever feel tempted to get "inspired", bad joke I know :).
Anyway I tried compiling my absolute favourite Rahman song list, not that all his songs are not equally great but just that these songs leave me speechless every time I hear them. Not in any order just pure magic.

• Nahin Samne Yeh Alag Baat Hai (Taal)
• Dil hai chota sa choti si asha (Roja)
• Bharat Humko Jaan Se Pyara Hai(Roja)
• Tu Hi Re (Bombay)
• Ae ajnabi(Dil Se)
• Mitwa (Lagaan)
• Chale chalo(Lagaan)
• Ek Tu Hi Bharosa(Pukar)
• Pagdi Sambal Jatta (The Legend of Bhagat Singh)
• Saanwariya Saanwariya(Swades)
• Yeh Jo Des Hai Tera(Swades)
• Pal Pal Hai Bhaari(Swades)
• Tu Bin Bataye(Rang de basanti)
• Rang De Basanti(Rang de basnti)
• Ik Onkar(Rang de basanti)
• Khoon Chala(Rang de basanti)
• Luka Chuppi(Rang de basanti)
• Roobaroo( Rang de basanti)
• Tu Bin Bataaye(Rang de basanti)
• Mayya(Guru)
• Azeem-O-Shaan Shahenshah(Jodha akbar)
• Jashn-E-Bahaara(Jodha akbar)
• Khwaja Mere Khwaja(Jodha akbar)
• In Lamhon Ke Daaman Mein(Jodha akbar)
• Mann Mohana(Jodha akbar)
• Vandemataram (Vandemataram)
  
• Chanda suraj lakhon taare (Gurus of peace)
  

Here is link to you tube video, very old Rahman's interview from the "Roja" time.