There's a lot to choose from. I've used the Eze-lap wallet size diamond plates for the last ~25 years.
"The best" would depend on the type of knife, steel, edge, and use; weight, bulk, and other individual preferences and limitations, so there is no "one-size-fits-all" answer. Since I usually carry everything in the wilderness, light weight and low bulk are important. I like the DMT Double Sided Diafolds, especially the Coarse/Extra Coarse, for general maintenance. If the Baryonyx Manticore ever comes out in pocket stone size I'd also carry that for the tougher jobs.
"In the field" I prefer a double sided leather strop, made from a wooden "paint stir stick" and coated with 3 micron CBN on one side and ,.5 CBN on the other.
Your question is way to generic and vague to give a good answer but I use a small black Arkansas stone, probably about 3 to 4 inches long and less than an inch wide it gives a good finish.
