Hypixel is the best minecraft server right now. You will still find modders in about 60% of the games you join, but the gameing aspect of the server is still amazing and hard to beat.
I used to like just Factions, and that was Cosmicpvp. Then I realised there was loads of servers out there, I've played hypixel a few times, but always ended back at factions, but I do like to play new servers, rather than big established ones, as they look after you better. So I suppose I do not have a favourite server, just a preferred game, Factions.