You're going to get lots opinions on this subject, but the bottom line is, a block of wood will do just fine. Put it behind the wheels on the down-hill side. Really, that's all that's necessary. A concrete block will do the same thing.
Fancier is not necessarily better. X-chocks aid in some trailer shake, but there is always the problem of tire pressure increasing and decreasing as their temperatures rise and fall. (Personally, I don't use them). Maybe I'm a bit old school, but I want my chocks to be touching the ground.
I have used
these (click here) (same ones) on 3 different travel trailer now. One size fits all, and one set for each side of the camper. I've never had trailer movement using these, on even some of the hilliest campsites I've set up on.