To me, the caves of Aggtelek and Slovak Karst are the best WHS we have and I'm very proud of it. Depends of course on the cave you visit, but if you had been born, let's say in Eastern Slovakia, you would have visited most of them by the time you've finished your compulsory school attendance. Summer is the best time to visit, when there's a season. Most of the caves are closed off for winter, one or two are open. I would recommend visiting the Domica cave, either from Slovak or Hungarian part. Hungarians have it opened all year round and are much more accomodating than my compatriots are, I am sad to say. There's an underground river you can ride and you will cross border and the interior decorations are pretty. Another stunner is Dobsinska ice cave. Ochtinska Aragonite cave has beautiful decorations and colourings. If you manage to find yourself skiing in Jasna and the Demanovska Freedom cave is open, do see, it's also lovely.