Without the crystal ball I would suggest the least risky island to book is the one that currently has the strictest entry requirements, self-isolation and testing requirements at the moment. The stricter they are the less likely the disease will spread on island so by the time you visit they will still hopefully not have high numbers on island (which could lead the governments imposing a tourist ban). Difficult. Personally I'd plump for Barbados a) because it's my favourite and b) it's the island I'd trust to manage things best.