Sanctuary is
probably the best place to build. As someone else mentioned, you'll be hoofing it back and forth to Concord. That said, Starlight is not terrible given the caveats, and unless you're playing on Survival, you can fast travel. The benefit of Starlight is the close proximity to Drumlin Diner. That is, if you sided with Trudy over Wolfgang, that gets you a vendor who will give you a discount. Of course, so will Trashcan Carla if you respond to her introduction with "Just lookin' for love, sweetheart." (This works with either Nate or Nora.) And Carla doesn't travel to Starlight, until you unlock the caravan outposts from Bunker Hill.
Sanctuary is tricky because it's not flat. The trick is, of course, to use the plot types with foundations. Except for interior plots, all plots have foundation options, as far as I've seen. I tear down all the Sanctuary houses you can't use. The ones just north and south of the workshop house, I use for residential. I can get four 2x2 plots right on the foundation. The trick is to bury the foundation of the plot under the existing foundation, but only just, so everything else appears to be mounted on the house foundation. Why use foundation plots in the first place, then? Because they don't fit perfectly. Two will hang off a bit. So, you get a bit of a 'boardwalk.' I place industrial plots on the northernmost house. Keep in mind north of this is a raider spawn point. (The other being the bridge to 111. Tall scaffolding, 1x1 scaffolding floor, turret. Problem solved. Well, mostly. Upgrade it to a heavy laser turret when you get Gun Nut up high enough.
Then it's problem solved.) The ruined house southeast of this, I use for municipal plots, and residential plots go on the southeasternmost plot. As for agricultural plots, I put them in the backyards of the Sanctuary houses. Use the foundations there, too, so your dirt piles don't float. Instead, you get a bigger dirt pile that looks like crap at first, but somehow over time it blends in and looks like it was always meant to be there.
With Starlight it's easier. Just build wherever.