This is my first season here in Utah, but as a beginner rider myself I have ridden at Snowbird and it can be pretty tough. I've heard many people that agree. Other than that I'm not really sure.. I'll be at Brighton this year and they have a discounted 'beginner' lift ticket that covers two of the smaller lifts that are all green and blues. I would guess Canyons and Park City would have to have some good beginner areas because they're know as more touristy resorts so they have more beginners running through there.
I'm sure any of them would be fun though. I think it should depend more on what time of year you're coming, budget, where your'e staying, etc.
They're around SLC but it's a longer drive to Brighton, Solitude, and Snowbird if you're staying in Park City instead of in SLC because the road connecting park city to brighton closes in the winter which means you'll have to take I-80W to I-215 S to get down to the cottonwood canyon resorts.
How much of a beginner are you? If you're super new and will be sticking to just a few green runs it wouldn't make as much of a difference to visit several different resorts than just staying at one. That's just my opinion though from my first couple times snowboarding. I didn't really care where I was, I was learning and having fun.
You're right, most of them are by SLC. Since I'm from Vegas, I'm more familiar with the "ones by St. George", which are Brian Head and Eagle Point which is in Beaver. Both would be fine for beginners. Brian Head in particular has a great bunny hill that allows you to move through a wide range of beginner levels before getting bored.
Also, damanb is right, no matter where you are, if you're out there, you're gonna be having a blast. Once you get to the point where you can get down a bunny hill without falling and wanna try something bigger, Brighton in SLC has a nice area full of long green runs that you could spend at least a day on.