You can remove the barrels from the drive-in to remove the radiation.
The drive-in is my choice base. I have 3 buildings around the pond connected by bridges, with rooms of varying quality, and also an indoor mall and crafting area. There is one additional bunkhouse with garden plots on the roof.
I have walls on 3/4 sides, with a main gate on the road entrance with trip-wires and flame traps.
My personal house is balanced on top of the screen, with overhangs I can watch the settlement from, as well as jump down in power armour.
We also have a small arena near the kiosk for fighting captured creatures.
And finally we have a bar/dining area next to the kiosk where we gather at night.
I have used the exploit to increase the size limit several times.
My second choice has been Warwick Homestead, as the options for defence were obvious.
My brother uses Spectacle Island, and has a nice setup, but the untamed geography and some annoyingly un-removable buildings provide some challenge.
The airport would be great if they hadn't prevented you from planting food, even with planters.