Mine have curved blades but that doesn't mean those are best. They're just what I have and am used to. The one with the hole in it is much like using a round knife with the points cut off. I like the control I get with it. I have used straight blades, too and they work fine but my preference is a curved one. The angled one is good for running along an edge and the other is good for other thinning tasks like the end of a strap or belt. I suspect that there isn't any best one, just what you get used to and like using.
If I didn't have a good knife for the job, I'd try one of the Osborn leather & skiving knives or one like them. I think the Japanese make some pretty good knives for the money. The main thing is to keep it sharp so there's no drag when you cut with it. Ebay would be a good place to look for used ones. Amazon lists a bunch of inexpensive imported ones but there's no guarantee those will be good enough steel to properly hold an edge.