This is what I do/did to get my land value to over $80. Maybe it is a correct combination of parks? However I often have park radius's overlapping each other for the specific reason to raise land value sky high.
I've never had a single park produce negative land value.
I will have to look at them again
I think I did not say that correctly. Of course, parks have a very positive effect within the radius they show upon placement. And yes, you should have those radii overlapping, as this potentiates their effect. It is just that the area of the park itself doesn't have high land value, and this seeps out over the edge, just a few cells. Which basically just means that houses on the park edge seem to sit on slightly lower land value than houses a few cells away, all just assuming the graph is actually correct. (which is not a given). So, if you put a park next to a river and have a house in between, it probably sits in a blue area (see for an example in my image near the river bridge). You can also see the bluish green halos around parks on the top and slightly below that. That's what I refer to. That halo is so narrow, that large 4x4 lots are less influenced than smaller lots.
On the other hand, I have such an overkill of landmarks in that area that this all doesn't matter much.