Investors may not have liked all of the news they got in the past year, but they can’t complain about their ability to retrieve, understand, and act on it.

Online brokers have talked up “mobile first” development for years, but in 2017 and 2018 they finally delivered on the promise of a more consistent experience across platforms—whether it’s creating watchlists, making charts, or following daily events—for their clients. Their success came through again and again as we reviewed the firms in Barron’s 23rd annual ranking of...