Which settlement is the best?

From my experience so far, I'm going to say Starlight Drive In. This is because of the following:

• It has direct access to water for water purification (the middle of the parking lot has sunk and has radioactive water)
• It has a lot of land, with some very good vantage points from the diner at one end of the parking lot and the sign from the other end. Plenty of locations with excellent visibility to install turrets, the rest of the parking lot is quite flat
• It's relatively centralized to the initial areas (half way between Cambridge and Sanctuary)
• It has enough dirt to plant food to fully maximize your population without any issues
• There is a lot of scrap (old cars etc)

Spectacle Island is the biggest potential settlement in the game. It's surrounded by water, has the most dirt, largest build area, and the highest building height. The Island also comes with the largest power source in the game. The prebuilt generator gives 20 units of power, however 5 power is used to run the pulse beacon to repel mirelurks. Look for "The Castle", the Island is off in the ocean. You can build anywhere on that island. The layout of the island is a bit messy and with the hills it can be difficult to build, but it's about the size of all the other 20 or so settlements combined. Seriously it's huge. Currently I doubled the building budget on my game with Console and have a huge fort there.

Now for the best area I would have to go with Starlight Drive In. Very flat build area, plenty of salvage, centralized water hole (you can salvage to radioactive barrels in the water), and has nice vantage points. The building budget isn't the biggest but is moderate. It has enough dirt to run a maxed settlement of 21 without issue.

Sorry for the long post guys lmao. I just love this new feature even though it could be better :)

Sanctuary Hills is not centralized, but hits a home run on all the other criteria. Of course, centralization is largely irrelevant.

It also has the advantage of being immediately available, meaning that you can start building your settlement before you go anywhere or do anything else.

In addition, it starts with all the crafting stations already built, making it even more useful at the start of the game.

Note that you get experience for building things both in workshop mode and at crafting stations. On one playthrough I leveled up twice before even advancing to the Red Rocket Station.

Also, Trashcan Carla is usually present, making it easy to trade here as well. If she's not around, sleeping for an hour usually makes her show up for me.

It's also defensible, with just a few key entry points.

On to your criteria:

Biggest building area.

If you need more area to build than Sanctuary Hills provides, I'm not sure what you're doing.

Most water.

There is a river surrounding the settlement, with room to build many, many industrial water purifiers.

Most dirt for crops.

There is plenty of nice dirt all over the place. I usually have a nice garden going near the entrance road, where there is already a cooking station set up for you.

Flattest surface.

Unlike a lot of places with almost-flat surfaces, like the Drive-In, Sanctuary Hills has large totally flat areas after you scrap the ruined houses. Using these platforms saves you resources otherwise spent on building foundations. In addition, there's quite a bit of almost-flat area you can easily build foundations upon.

Centralized location.

Sanctuary is not centralized. This would be the biggest issue given your criteria, but a centralized location really isn't very important. You can fast travel right to it.

If you're concerned with fast travel times, perhaps to maximize a sleeping bonus, then you should gain access to the molecular relay as quickly as possible, and maintain it for as long as possible. Using the molecular relay only takes a minute, so you can travel anywhere in the Commonwealth in two minutes.

Most scrap available.

There is a lot of scrap here initially. You can scrap the ruined houses, as well as items laying about in the not completely ruined houses. This provides more than enough material to construct a water purifier, generator, and recruitment beacon as soon as you enter the town.

Using the pre-existing homes also saves you a lot of resources that would otherwise be spent on building structures for beds.