The best months to visit Colombia are between December and April when most areas tend to be at their driest.
In the Amazon it rains heavily throughout the year, with a slight respite in July and August. The Choco, on the Pacific coast, is one of the wettest places on earth so expect rain throughout the year.
December, January and Easter get very busy, as locals like to travel at this time of year. January is also when the famous Hay Literary Festival takes place in Cartagena and later in the year (August) you have spectacular flower festivals in Medellin.
Whale watching on the Pacific coast is possible from June to September and the water temperature is generally warm enough to swim in both the Caribbean and Pacific all year round.