I've sessioned in dungeons in England, the Netherlands, Belgium and Germany (where most of them happened). Germany stands out with the quantity and quality of "toys" they have, plus there are often plenty of Dommes to choose from since it's entirely legal and mostly an accepted part of the culture. In many ways the Fortress is like the dungeons in Germany in the number of Dommes and the quality of the equipment. In Germany, unlike the other countries, you almost always know the address of the dungeon from their web site, which is useful if you're traveling to a city that's new to you.
The most impressive dungeon I've been in is Studio Avalon in Berlin