the "best" is an arbitrary term, as there are a LOT of printers out there that you can utilize to make coroplast signs.
if the HP R1000 is in your price-range..... pull the trigger, and don't look back. Currently DYING to get one myself, but I just bought a new HP365 and my 2nd CNC router but the R1000 is at the top of my list for purchases next year.
not only would you have the ability to directly print to your substrate, which saves a TON of cost/effort, but you would have the versatility to print to rolls of film if you branch-out to car wraps, wall murals, & misc. decals/graphics.
besides, that machine is made to be user-friendly and easy-to-maintain. if you're not printing a lot of white, you pull out the white printhead and run the machine without it while it sits in a rotating drum and keeps the ink from settling within the printhead, and cycles the white ink for you. Head strikes ruin a printhead? no problem, plulg&play a new one in. Heck, it will even align itself for you. Granted, with ease of service comes some pricey serviceable parts expenses down the line, but time is almost always more valuable than parts & downtime.
I would highly recommend HP products to anyone getting into the market for the first-time. They really do make their products as simple as possible to operate & maintain.