From personal experience I'd say advertising on Facebook & Quora. There you can get really deep rooted advertising campaigns targeted to just the right users. I'm forever getting ICO ads pop up on my feed, but that's because Facebook understand that I LOVE crypto. It even knows what kind of crypto projects I'm likely to invest in, and what one's I'd probably throw away. Lately, I've only been seeing really worthwhile projects appear on my feed.
Quora is good because you can target questions - and there are millions and millions of people on Quora that use crypto and asking questions about icos.
I'd also recommend reddit advertising from what I've heard others say but I'm definitely not talking from experience there.