1. Twitter
2. Reddit
3. This forum
4. Telegram
5. Slack (which i heard is full of scammers)

All of the above I would say

The point of promoting an ICO is to reach as many people as possible, the more social media you will use the more users will potentially see your message, as not all of them user the same platforms. Telegram is usually dificult to advertise as you get banned, but the rest is quite convenient and you can reach quite a big target audience. I like especially the forums because you can attach pictures and link to websites etc., which you can do on other platforms as well, but in forums especially Bitcointalk.org it's really simple to present the project and idea simply and clearly, there are many customization options and that makes the project looks all that more attractive.