aaj rapat jayein to hamein na uthayiyo... humein jo uthayiyo to khud bhi rapat jayiyo from namak halal is also good one... though the most sensous of them all is tip tip barsa paani ;-)...
Just remembered another one, "Ek ladki bheegi bhaagi si" It has to be the best rain song ever, Madhubala and Kishore kumar in chalti ka naam gaadi, I would rate it over tip tip barsa paani in sensuality and as a song. Link to this song on youtube: http://www.youtube.com/watch?v=hTO1_322aYY
I also like 'Saawan barse tarse dil '..from the movie Dahek featuring Akshaye Khanna n Sonali Bendre... I don't think most of you must have heard it though..