Blitz vs Rapid
It depends on what attribute you are measuring. Whether for improvement or for enjoyment.
To break the two votes for rapid, I choose blitz as I am able to sneak in short games after a long day of work (though I have stopped playing for months). This is coming from someone who is a working adult and cannot spare much time for games. In addition, I play rapid only during over-the-board games as I somewhat do not trust players online for rapid games unless the players are already local chess club players of mine.