I think all answers are correct. Personally, I like more both this one and substantial talk (from @Elian) because I can say: "I like insignhtful talks" or "I like substantial talks". If I say "I like serious conversations" or "I like discussions" or "I like conversation" or "I like to talk", it sounds less clear for me.

I'm not sure of the meaning of "heavy conversation" @FumbleFingers: I just asked for the meaning here: http://english.stackexchange.com/questions/167553/what-does-‌​heavy-conversation-e‌​xactly-mean