Amsterdam of course...but here are some fun options as well:

Maui! (Or anywhere in Hawaii you are guaranteed to find good herb)
Humboldt County, CA (always someone willing to share good herb there and man those Red Woods can tell some amazing stories...lol)
Vermont (nice crunchy communities and beautiful greenery)
Any Rainbow gathering is fun.
Tempe or Sedona Az is toker friendly with no short supply of weed...but gotta watch out for the law.
You can hike the hash trails in Afghanistan...but of course the risk is great.
Have fun and Peace!