Well, I can't take the 'low road' here, heh... I have to be honest. There -was- a lot of good experience that came from being a JW.
For one, we took a long trip when I was a kid (1983, I think) from SoCal to the midwest. My mother wrote JWHQ and asked for a list of congregations on our path, then wrote the congregations to find volunteers for us to stay with or ask for help if needed along the way. We traveled in an old Ford van which DID have trouble a few times, including blowing a head gasket in Amarillo, Texas. We stayed with a really sweet family for 2 days while they fixed it for us. Every place we stayed, we knew we were safe with decent, good people.
Besides that, I liked the assemblies. Growing up in Southern California, circuit assemblies were always held in Woodland Hills - man, that was such a neat building, and its a crying shame its been torn down now. Often, instead of Dodger Stadium, we'd visit family & friends in the San Francisco area for the district conventions, which doubled as a summer vacation for us kids. (Granted, long talks were boring, but the dramas were pretty neat to watch, especially before they streamlined everything to make it all so universal...)
Sadly, when I get invited to a party at someone's house now, I cringe at the thought - growing up, you always knew a 'get together' would be a friendly place without people getting drunk and acting ridiculous, or a fight breaking out, etc. There's something to be said for that level of morality, really.
And to this day, I'll still have a random Kingdom Melody or song pop into my head. Also, being able to speak in public without any fear, and being able to organize material into a presentation - both good skills for management positions later in life, heh.
Don't get me wrong, none of it is worth going back... but the question was 'What was the best thing...' so there you go. :)