Sunsets in Santorini are spectacular and watching the sunset is probably one of the most popular activities on the island.  You can get a good view from a number of places on the island but one option is in hand gives you a view across the caldera to the sunset. Many bars and restaurants in Fira also have views so there are a lot of options. 

Another option, but a little pricey, is the sunset cruises.  There are a range of these from smaller catamarans taking 6 -10 passengers to much larger boats with 50 - 100 or more.  They all jostle for position at the entrance to the caldera but do provide a good view.

A must see sunset is in the village of Oia (also written Ia) on the tip of the island.  Local buses travel there frequently from Fira and a taxi ride is relatively cheap.  There are no vehicles in the centre of this picturesque village, perched on the top of the cliff.  About an hour before sunset the streets are packed with visitors and to find the best spots, just follow the crowd.  Many of the restaurants and bars have good views and you could try the roof top bar of the Pelican for an uninterrupted view. Another option is the bar at Fanari apartments.  The bar is small but is in a great location.  Asking the locals is the easiest way to find it. 

 Probably the best view of all is from the ruins of an old church right on the edge of the caldera in Oia.  Hundreds gather there every evening.  Take your own provisions as there are no bars or shops close by but it is worth the trek and the wait.