I tend to switch between them a lot, but only during certain situations.
I use the Automatic ring when I have well over 100 rings to spare to go on a frenzy, and aim at incoming opponents, or the ones that tend to pass by me. I also use it for long distance shots as I have a better chance of hitting the opponent with more than one shot upon firing.
I use the Explosion ring when the opponent is at close-medium range. The way I use it, is to fire it either directly at them, or right in front of them (considering it has a good blast radius).
Rail ring is for those long-distance shots, or for those who just run in a straight line, as you can kinda tell where they're going to go. I also use it for surprise shots, as nobody really expects it to be used at a close range.
The Scatter ring is probably my favorite. (If not second favorite, with Explosion ranking at the top) This is the one I use in ANY situation - long, short, medium distances - you name 'em. It's good for all-around usage, so long as you can at least pinpoint your opponent near the center of your aim.
The Bounce ring is the one that I don't use much, unless (as Mystic has already said) it is a tight space, and I can easily reflect it off of the walls (i.e Sapphire Falls CTF bases). Other than that, I don't use it very much.
And finally, the Grenade ring. My personal favorite for just dumping it behind me and having people run right into them. That or setting them up in front of the CTF bases (heck, maybe even inside), and having people get hurt by them willy-nilly.
So my three faves are Explosion, Rail, and Scatter, pretty much.