I've caught a few shinies through DexNaving and I would say that any Pokemon is a good start, however there are a few that to try and DexNav more than others here are some that I recommend going for. I would say that any Pokemon that fits your interest in terms of shiny is what you should go for. First, I would say stay away from DexNaving in water or caves. The problem with DexNaving in these areas is that the Pokemon tend to move around a lot in these areas, sometimes to the point where you will barely have enough time to sneak up on them so I don't recommend going for these. Unless you have the patience and persistence, stay away from DexNaving Pokemon in these areas. Second, is that I recommend DexNaving for Pokemon that can be found near a lot of grass, two examples would be Route 101 and Route 116. I find that areas like these usually tend to have a higher chance to spawn a Pokemon through DexNav. And while this may be due to location or Search Level, I think it would be more on location, I found that some Pokemon are easier to DexNav. During my time of DexNaving Zorua, Eevee, and Vulpix, I have gotten some results that differ in all 3. I would be able to force spawn a Zorua through DexNav most of the time, Eevee almost all the time, and Vulpix I would rarely be able to force spawn. Another tip is that I don't recommend going for Pokemon that can be found on both land and water and be near each other. An example of this is Marill on Route 117 as there is a chance that the force spawn can also happen in the water in that area.
So in general, I would say to find a Pokemon who can be found in grass, is located near a lot of grass, can't be DexNaved in water or can't spawn in water. I hope this does help and if you have any more questions, please let me know :)