I'd suggest Starlight Drive-in or Taffington Boat House.
The drive-in is quite large, and has a water source--I prefer settlements where I can build a purifier instead of pumps. (To get rid of the radioactivity, just scrap the barrels in the water). The area is largely broad and flat, making it easy to build on, and there's loads of scrappable stuff to turn into building materials. The screen and diner provide indoor/sheltered areas, and the rest of the area provides plenty more space for building. It also is fenced in--the fence has seen better days but is at least some standing testament to protection and is a start if you want to add to it (or alternately it's another scrappable source of steel if you don't want it). I'd say the only downside is that the farmable area is fairly small, but I find it's enough to keep the settlement fed. Finally it's in a relatively central location.
Taffington is small, but is in a decent location right along a road (you can build defenses right along the road), and there's water on one side so it feels defensible without having to spend a lot of time building walls and stuff (if you like to do that kind of thing). You might love or hate the non-scrappable house there (especially as the second floor is missing its roof), but I think it is more room than the gas station and you can also build on top of it and the boathouse. The boathouse is a good site for a purifier and the front yard has ample space for crops. It may feel also a little cozier for a companion house while still providing wiggle room.