Doug Bragg Enterprises makes the best mower I've used for rolling blueberry ground.  It is also by far the most EXPENSIVE mower.  Triple-gang with three 3' heads will run you over $35,000.  It's hydraulically driven and requires probably a minimum of 70hp, preferably 80+hp.  Sucks fuel like crazy compared to a mechanically-driven triple-gang such as the Maine Blueberry Equipment ones made from Vrisimo 3' flails but is much more robust. 

 For a smaller grower who doesn't want to spend huge bucks on a mower I'd recommend something like an Alamo (used to be Mott) super heavy duty.  Lots of these around on the used market.  Get something along the lines of a 5 or 6 footer instead of an 88 (we've got two SHD 88s here) and mount wheels or rollers along the front tube near the 3pt mounting points.  That'll allow the mower to ride the knolls better without running the cuttershaft into the ground.  We got along well without the gauge wheels with careful operators on less than horrible ground, but we've also bent two cuttershafts over the years.

 Kverneland makes a good mower as well (they bought Malletti, so you're basically getting a Malletti flail).  I bought one last year from Fort Equipment in Amherst, NS but can't remember the model number offhand.  Tell them you want it for blueberries and they'll get you the right model with rock knives.  They sell lots of them there.  I'd recommend setting the gearbox to rotate in the same direction as the tractor wheels rotate (I think they come the other way by default, but it's a simple swap in the gearbox).  Cuttershaft contact with the ground is a bit less destructive that way.  You can buy premade brackets from Doug Bragg Enterprises to mount their mower wheels onto the front of the Kverneland to make it follow rolling ground better.  I bought the brackets and wheels slightly after the mower and it got sent into use without them.  Works great, but after getting into much rougher ground the operator bent off a few of the ears on the cuttershaft (weld-on clevises) that hold the pins, d-rings, and knives on.  The Mott/Alamo design is stronger on rocks, but I think the Kverneland does a better job when using the exact same knives on both.  You can buy the weld-on cuttershaft ears even if the parts guy tells you they aren't in his parts book.  He might have to call the factory to order them but they can be bought.

 Tardiff makes a good big heavy mower but I have no personal experience with them.  Maschios are good for mowing lots of larger bushes but not so good after the bushes are gone and are very expensive to run through rocks.  Mowers with smaller knives like the Bragg, MBE, Mott/Alamo, Kverneland, etc are the bread-and-butter blueberry mowers.  As long as the ground isn't too dippy, I prefer a Kverneland or Mott because they're cheap, simple, good, and transport from field to field easily.  Triple-gangs are more difficult to move and we do a lot of field-hopping.