The standard cylindrical radius on Martin guitars is 16", Taylor is 15".
Larrivée uses a compound (conical) radius of 16"-21" or 17"-21" depending on the model.
Standard classical guitars are flat, but I have seen so-called "cross-over" nylon string instruments with non-flat radii. I couldn't quote the numbers for you on this.
As the esteemed Mr. Ruskin has said, there is no "best", nor is there an "easiest" radius (whatever that means)...it's simply based on what you prefer...and of course, what you prefer can only be based on what you've experienced first-hand (no pun).
Most stores (owners and employees) will have no idea what the radius numbers are for any guitar hanging on the wall...and having a radius gauge may be of little help, since most guitars have strings attached!
