It really depends on what you’re going for. I like a 24-70mm f/2.8 zoom lens when I do magazine food shoots because it allows me to work around the scene. I can use the wider focal lengths to capture overhead shots of the food in its environment. I can use the longer focal lengths to make the hero shots and show details. I’ll also typically bring a macro in the 50-100mm range in order to show really tight details. If you can only pick one focal length, I think 50mm is the way to go. You can crop the image if you want to go tighter, but you can still realistically hold it up over a table and take a shot of the scene without feeling too tight.