As said above definitely the islands as they have at worst 4 but most of them seem to be 6 base tax. Brazil is reasonable but a few provinces down is a region that has 5-6 base tax area's.
(Rio de la plata, Banda Oriental and the surrounding provinces.)
Would go for Brazil first though just to deny the AI and you probably wouldn't have discovered the provinces under it quite yet.
Central america isn't that great, take the coast and a bit in lands around Delaware and Manhattan, 4-6 base tax.
(Note: This is however from the demo, so it is subject to change)