Re: Fidelity, by excellent I mean: it does what I want it to do, and the UI is clear enough. To be fair, I'm an infrequent trader who mainly uses Fidelity for banking, so I don't spend much time on the app. I'm also willing to sacrifice UI flashiness for a functional service that doesn't block me from trading when I do.
[ETA: I believe there's a lot to be said, and has been said, for ugly functional UIs that outlast fancier competition. The archetype is craigslist]