I just checked the optin form stats on my blog after reading your question and search engines win
For me it's search engines, social media (mostly facebook, twitter, youtube and google+) then followed by tumblr, stumbleupon (even though I don't have a stumbleupon account) and other sites.
Even though search engines are my number 1 traffic source, I was expecting social media to be number 1 in terms of conversions because I get a lot of engagement on social networks. But numbers don't lie.
I can't vouch for paid traffic because I use it once in a while, I rely on free traffic. But looking at my stats, if I were to include paid traffic then it would be in this order:
1. Search engines
2. Social Media
I'm really good with social media marketing that is why I can get better conversions doing social media marketing. Some people may argue and say paid traffic can be 2nd (better than social), but if you look carefully you'll realize that they're not good at social media marketing.
Most marketers can't even get 10 post shares. I'm not talking about likes, comments or blog shares. I'm talking about the shares on each post that is on your social profile/page. So I don't think it's fair to say paid traffic is better than social if you can't do proper social marketing
So I hope everyone who answers OP's question does a fair analysis.
And hope that answers your question OP.