Twenty-two apps. Honest note: "brain training" research is mixed. You get better at the specific game, not general intelligence. That said, some apps have domain-specific benefits.

Well-researched (6)

Duolingo, Anki, Brilliant, Khan Academy, Coursera, Memrise.

Popular (with mixed evidence) (10)

Lumosity, Peak, Elevate, CogniFit, NeuroNation, Happy Neuron, Fit Brains, BrainHQ, Eidetic, Mensa Brain Training.

Specific skills (6)

Chess.com, Lichess, Sudoku, Wordscapes, Crosswords (NYT), Lightbot (programming logic).

Language learning + domain-specific skills > generic "brain training." Pick one from the first section.