Avoid Columbian bananas - they grow a variety that tends to turn grey and rot rather than ripen. Supermarkets buy them because they're the cheapest available. They're quite simply awful. Costa Rica and Dominican Republic bananas are better, but best of all are Windward and Leeward Islands varieties - though you'll have trouble finding them in supermarkets. In those far-off days when supermarkets sold ripe yellow bananas, bunches would have have been hung in a warehouse in cool and dark conditions for a couple of weeks before being divided up and placed on shop shelves.