Ok, so I've wanted to know the answer to this too, and was training up my Pokemon for the league and at the same time filling my dex by evolving the pokes I needed to evolve, and then got bored and wanted to do a small amount of poke finder grinding... and I found what my favourite poke finder spot is, I can consistently get between 5000 - 6000 thumbs up most photos... and it just so happens to be on...
You can mainly see vanilite and vanillish here, and, although vanilite sometimes might not face the camera, if it is it smiles very often making it easy to get more than 3000, but vanillish is what I keep finding over and over, and they always smile, meaning catching them facing you with the best zoom you have available can get you good points, I'm on level 4 meaning I have the max zoom and I always get 5000 - 6000, and many times they just slowly float along. Sometimes they move very fast but if you get a good photo it shouldn't matter. And conveniently there is a big blue ice crystal thing that it can start moving directly forward (so facing you perfectly) at so just line the camera there and you should get a good shot.
It's all up to preference, and I haven't seen too many spots, but this one seems pretty good, try it out and if you like it too then great, whats not great though is that either way it still takes heaps of grinding :/ oh well. (who thought it was a good idea to have level five be 1.5 million points)