Napa is one of my favourite lads holiday places, been back 3 times since my first venture out there when I was 17!
As Oli says above Ibiza has great night life but you get stung with Expensive Big club entry and drinks (€8 for a small bottle of water, and €12 + for beers. If you get on the mixers it gets into daft money!) I was out there for 3 nights in June and some of the lads did £600 just on booze/club entry while we were there.
Malia is always lively this time of year, more like bars than proper clubs like Napa and Ibiza but they are busy and full of skirt!
Kavos is Ok too but beaches their arent much kop.
Kardemena (Kos) is only very small with 2 main clubs and you have to go out early as Bar music is turned off at midnight.
Zante has a few mini clubs/bars that are good after a few beers, beach is average but better than Kardemena and Kavos beaches in my opinion.
Ive not done Faliraki or the Canaries Islands (Yet) but from where I have been I would say Malia or Zante.