That depends a lot on how you want to tell the story. Most of the time the primary protagonist is the viewpoint character, because that’s right in the thick of things and is much more exciting. However, taking a side-neutral character’s perspective can be interesting. That’s the squire (Pagan’s Crusade, etc.), the unrelated third party (Imagine Harry Potter told exclusively from Neville Longbottom’s POV) or even the Wise Mentor’s perspective (Imagine The Hobbit told from Gandalf’s POV).

Really, choosing the protagonist and viewpoint character is dependant on the story and plot. Whose POV will be more intriguing or enjoyable to read? Whose ‘voice’ will readers attach to best? And more importantly, which protagonist will move the story along best? It’s very much a different process for each individual story, depending on plot, setting, available characters to choose from, the writer’s mood and even cultural norms/ expectations of the reading public.