It depends on which version of NAV you are using and what you are integrating. Transactions, documents, master data or? Online or batch?
If on a newer version of NAV (2016+), then I would base it on the data exchange framework. Then I didn’t need to decide on the actual data carrier, which could be web services as suggested by Gary. The DEF is a very power full import export, often overlooked by many, as it takes a bit of time to fully utilize.