hi,

see its totally dpends on interest you have got after studying 4 years of btech course, but you also need to think about the current technology trend and scope of your mtech courses in job market, to do mtech iits, or iiits is the best for placement point of view. now some of the mtech courses related with ece is,

rf and micro wave technology,

communication engineering

digital signal processing,

embedded systems,

vlsi and microelectronics