Hi ramya

Are you asking topics for technical fest or design of ppt you would like to know

In case of topics means i would like to suggest you that consider your branch which you deal up , make sure you choose wisely as per the new technologies on platforms 

Let me tell you few best booming technologies

Big data hafoop, ROBOTIC PROCESS AUTOMATION , THERMISTORS , SPINTRONICS ,AUTOCAD lot more

While if you wanna know way for preparing the best ppt then make as simple and attractive ppt with decent colour , most importantly dont put information in paragraph put them in points wise thats best 

Use as many pictures as you could , since pictures conveys lot info rather sentences 

Use graphs to proves and strategies 

You will even find best ppt template online find them

Hope this helps