I am building a Racing quad completely from scratch so decided using Banggood to buy all the equipment.
But I read many threads on this forum which pointed out  that customs are painful to handle and would cause issues! 
If i order motors, ESCs, Tx&Rx, flight controller, the frame, which  of these is most likely to be detained at customs? What stuff should I order from Banggood to keep the cost of the quad as low as possible?

I am also building a fpv racing quad, I bought every part from banggood or aliexpress. Motors,esc,flight controller,frame,reciever,camera,video tx etc.
The thing that I would not recommend you to buy is the radio transmitter, because the box is usually big and it catches custom's attention.

#tushar

But if we buy tx from LHS or within India it costs double so from where do you buy cheap

I recently bought a Frsky  Taranis qx7 from rcmumbai.com for rs 12000, in other countries like us it is around 7000rs.  Rc items are definitely expensive in india.
But the taranis is a great transmitter and one time investment and will serve you for years if you take care of it. Or if you are limited by budget, you can buy the flysky
Fsi6x in india. Search on quadkopters.com , rchyper.com , robu.in etc. It's price should be around 4000-4500 rs (2800 in us).

I need to buy the flysky 10ch transmitter from banggood. So will it be caught by the customs in India ?