I had a machine shipped to me on R & L Carriers, it arrived just fine. But I have seen negative comments about other people's experience with them. I think a lot depends on how well the machine is packaged by the place that ships it to you, if they use a crate or if on a pallet do they wrap it well, etc. This is one of the difficult things about buying a machine, but if it is insured and something happens then hopefully the seller will handle the claim.
I'll echo what GOS said, there is a website called uShip that helps connect shippers to customers. The individual guys that do the delivering will bid on your shipping request, they have ratings (sort of like ebay) from previous customers. I've had a couple of arcade games delivered that way, each time worked out well.
There was a very popular shipping company that many people endorsed years ago, the contact person was named Michelle. I don't remember if it was NAVL (North American ValnLines) or something else. But they got pretty expensive to use.
I wonder what shipping method IGT or Bally uses to ship new machines to the casinos nationwide when they are ordered?
If you buy a machine from a slot machine resell company they may already have a trucking company they deal with exclusively and you won't get a choice on how it is shipped.