Generally information about the contests you can check everyday in Newsticker section on Tibia.com. CMs as soon as fansite started a new event announce it on the top of the official site. Some of fansites copy Newstickers and publish it n their sites/social media.
I think there is no sense to share info about current contests, because this questions and answers should be useful for other users in the future. Your sub-question is really short term, but well ... on TibiaMagazine you have current valientine's contest, on ViavaTibia you can desing their mascot.
About contest schedule, for now the only info are Newstickers on the top of official websida tibia.com. You have to check it everyday or check the recent history to see if there is any active contest announced.
TibiaBosses.com is working to add it as a new feature. Unfortunately I can not give you any exact date of implementing it.
Another thing is what you mean by "the most active" - how many people visit the fansite per day? Because activity of the fansite isn't only about organising contests. Info which fansites are the most popular among the users is avaliable for admins on fansites board. CMs making from time to time statistics for them.