but which signature campaign is the best?
It depends on the following
Rank - look up what ranks they are allowing mate, otherwise you cant enroll.
Payment - though payment should not be the biggest issue but in general people always flock into those which have highest payrates.
Manager - if its managed by some Green trusted and/or known person who has managed campaigns before, you can trust they wont turn scam (in most cases)
Your free time - since the campaigns should be done as a side earning, make sure you can spread the posts over the entire week and not finish everything in 2 days or rush all the posts on the last day.
If I were you, I'd search for affiliate programs. Especially those that have lifetime bonuses, not one-time
You need your own website and/or platform to spread your link into public and if cant draw traffic to that link its as good as nothing.