As you left no room for error by requesting the ‘best prime rib” on a Grilla Grills smoker, you leave me no choice but to break out the big guns: AmazingRibs.com. Meathead Goldwyn has a great write up on prime rib that I used to start mastering this expensive hunk of meat.
Through my years working with prime rib, there are a few things I have personally adapted. I like a little more char on the exterior of my prime rib, so I like to employ two cookers. The first will go for low and slow, and once my prime rib hits about 130 degrees in the very center, I get the second smoker up to about 450-500 degrees for a really good sear to set that crust.
If I am not using the two grill method above, I throw my prime rib on the Kong kamado grill. I just love the harsh contrast of charcoal smoke with the meat.
If you are using a Grilla or Silverbac I wholeheartedly recommend using a very strong flavor of pellets, like mesquite or hickory. I also recommend NOT burning cheap pellets here. You just paid upwards of $100 for a large chunk of meat, so get some high quality pellets such as BBQrs Delights. Your taste buds will tank me later.
Now for the seasoning and crust on the outside. I am a sucker for rosemary and thyme. Typically I’ll coat the prime rib with Tones Rosemary Garlic seasoning. You can find it in pretty much any grocery. It has a great blend of rosemary, garlic, and other spices to crust up your prime rib really well.
The second seasoning, that I have just discovered, is a product called Twisted Q Insane Steak. I’ve seen it at some big box stores, but it can be a little harder to find. It is just a touch punchier than the Tones product, but I think either works fantastic on this cut of meat.
Last recommendation: don’t cheap out and buy premade horseradish sauce. Make your own! The difference is vast and you certainly won’t regret it. Here’s a recipe I use, but I like to double up the horseradish and add one tbs. of lemon juice to mine.
Hope this helps!