The Watchtower Ruins is the best Culture Building in the game - assuming it is upgraded. Not only does it increase culture, and the value is a lot, but I consider the strait culture the secondary benefit. The real value of the Ruin is the increased neighborly help value boost - polished buildings - look for the hands at least on PC.
The sanctuary/martial monastery is the 2nd best Culture Building in the game - though 5 tiles larger than the watchtower - it also at its maximum provides 40% culture relative to the city's required culture. Unfortunately, this wonder has been pigeonholed as a troop wonder, which is unfortunate.
My City's Required Culture is paid for with my Watchtower, Sanctuary, and streets, which means if I wanted to, I could remove every other culture building in my city. Of course, culture buildings provide more than just culture, so I do not recommend doing so. However, culture provided by AWs and streets allow room for all the low culture seed & mana buildings.