I enjoyed the Sky Garden and its view early one clear fall morning, there were few people there, I didn't need a reservation, and they served a nice breakfast! Great view from the top of the Walkie Talkie that included the Shard, Cheese Grater, and Gherkin, as well as more prosaic places like the Tower, Thames, St. Paul's, etc.
Went up and down the London Eye on an earlier trip and liked it too, but the viewing time was limited, line was long, and no amenities (breakfast!) up there. It was a rainy day, though. My pictures of Westminster Palace look sort of like Monet's famous painting, lots of squiggly lines from the rain on the glass.
I haven't been to the Tate Modern, which isn't as high as the others, but I can imagine that the view from the roof is great.