For vanilla, I enjoy the big prison because it just swarms you over and over, which is great. I hope that isn't changing with these new crawl spaces.
The nursing home is really good. And I have always liked the skyscrapers.
The Grover High is good, but it just completes way too quickly. But it is still near the top of my list.
It is hard to pick lower tier POI since they are completed so quickly. But there are some good ones that stand out from others, like the daycare with all the colors.
Basically, if a POI has a lot of zombies per swarm (within reason... I'm not talking about 1000+ your POI) or is unique and makes unique and interesting use of blocks and painting, them it is one I'll probably like more than others.
I'm sure there are other POI that stand out, but I always include a lot of custom POI so there is better variety, which makes it hard to pick out of just vanilla POI. Plus, I don't really take time to rate POI. I enjoy completing all of them.