I can recommend the EXIT Festival in Serbia. I’ve been twice and the setting in the Petrovaradin Fortress is spectacular and unique. The dance arena in one of the old moats is especially awesome.
A great small festival that concentrates on electronic dance music is Beatherder in the north of England. I went last year and had a fantastic time, The stage in the woods is amazing. Gutted I can’t make it this year.
A festival I haven’t been to yet but hope to at some point is Hideout in Croatia. I keep hearing good things about it.