Five from me. It's very hard to narrow it down!
steemitblog: For keeping abreast of official announcements and developments and gaining an understanding what it going on with the blockchain, at least from the Steemit Inc team point of view.
smooth: He posts very rarely but his comments are always worth checking out. He's the witness with (I would say) the best understanding of Steem economics and the most willingness to explain potential changes. I don't always agree with his views but if you have followed enough of his comment threads and understood them then you'll have a great overview of how the economics fit together.
utopian-io: Not just for coders. Utopian is one of the largest communities on the Steem blockchain and Utopian-io has links into all kinds of interesting projects. If you've any interest in open-source, development, technology, or science then this could be for you.
yabapmatt; SteemMonsters, Steem bot tracker, Keychain, Postpromoter. One of the most active developers on the blockchain, pushing Steem to new levels. Always a good idea to know what he's up to next.
acidyo: A long-time Steemian and frequent poster on a wide range of subjects. Always with something new to say. Runs some of the most generous competitions on the blockchain.