I want to fetch PO, SO, InventoryItem from NetSuite and create ItemFulfillment, Receipt for multiple customers using my app. What should be the best way to go? REST, SOAP or any other?

Just depends on what you want. SOAP is better at bulk volume in my experience. REST is a little easier for the unfamiliar. A restlet is also technically an option, but probably more development overhead than you are looking for.

Thanks @Tyson Freeze

Coming to integration either using REST or SOAP, please consider the concurrency. If no. of requests are very less in 5 min bracket then REST is best option (real time scenarios). Also, please make sure to handle the below errors in your app. 1. CONCURENCY LIMIT EXCEEDED or relavant 2. TIME EXEED ERROR 3. or any other ERROR thrown by NetSuite. Same is applicable in case of SOAP as well but it is useful for bulk data. Hope it helps.

Thanks @Asav me