ICM the the one that best encourages me to make a list of films. It could be that I was just determined to do it at the time I joined, but it's a good listmaker, and my primary source for keeping track of what I've watched.
However, Letterboxd encourages me to write. It doesn't just give me an opportunity to, but it actually sets up its system that gets me to give a one or two paragraph review, which I've not done as much of in the past.
The essential list for me is ICM, but encouraging me to write is great.