Ive collected a lot of resources so Ive decided I'm going to start building a settlement. Problem is I'm not sure to do it on sanctuary or the castle, which one would you recommend, or if there is a different settlement you'd recommend then please say it!

There are lots of other good candidates. If you want a lot of pre existing structure to build from the castle is the best. If you want a blank slate there are better choices

I've done Sanctuary so I'd say castle. Much of it is built, you can repair the walls and i thought about covering the whole middle courtyard and making another level on top of that since you dont have to build many outer walls. I have my eye on a few over water which you can do in sanctuary. The island looks ok but is sparse. I still like the Drive-in too.

It really depends on why you're building a settlement. If it's for artillery, you'll probably get more use out of sanctuary until you have enough materials to build up the castle. If you're trying to get rich off purified water, I got to 999 water at Nordhagen Beach (though that takes a lot of materials). If you just want a settlement, and had to choose one place, I would build up The Castle first because it gets attacked more than sanctuary and could use the defense. This is just my opinion and if you get really into settlements you'll end up with more than 20 of them. The difference between the 2 is The Castle is easier to defend, but gets attacked more (this is based off my experiences not fact) and has a NPC that sells energy weapons/ammo. Sanctuary has a NPC that sells shipments of oil (trashcan carla).

TL;DR: both

starlight is great. pretty easy to build on the partial level based land.