What are you using it for? Blackmagic cameras are excellent, but it may have a learning curve to get the best footage out of it.
Can't really watch all that footage anywhere else but a few TVs right now, and the footage is going to be HUGEMONGOUS... so get ready to only be able to see it in a couple places and also spend lots of money on storage. I think SONY has a good one. It's also 3D.
OP, I've worked with a whole bunch of 4k cameras (Alexa, Redd, GH4, Blackmagic, etc) and it really depends on your budget. I think the Panasonic GH4 is fantastic value for money but it's quite pricey.
