My favorite butt plug is a homemade toy that I made using a Galileo Dog Bone by Nylabone (any pet store should carry it). It has a very interesting shape with a rounded end that puts just the right amount of pressure on my prostate. The other end has two "bulbs" that prevent the bone from slipping completely inside my ass (very important for a butt plug!!!). I found the rounded end was a little too blunt to insert easily, so I attached the pointy half of a small plastic Easter egg to it (Put the half egg on top of the Galileo bone, and put it in a balloon to hold them together.)
There are 3 size bones - the small is useless (for me, anyway); the medium size ("Wolf") is great as a general butt plug and for extended wearing; and the large ("Souper") size is perfect for serious anal play. I love them both for masturbating solo and also when fucking my girlfriend. Both are great while masturbating because I can massage my prostate just by rolling my hips back and forth. The two "bulbs" that stick out are slightly different sizes, so depending on which way the larger bulb is oriented (either towards my balls or away from my balls), and also depending on how I'm sitting, there's either more or less pressure directed towards my prostate.
I like having my girlfriend fucking me on top while I have the "Souper"-sized bone up my ass. Her weight puts pressure on my cock and abdomen, which squeezes my prostate against the bone. I have to get her all worked up beforehand, because I usually can't last that long before exploding in her when she's riding me. And even when she wasn't that excited, the sight and intensity of my orgasm was enough to put her over the top. (Now, if I could only get her to try one in her ass while I fuck her pussy ... THAT would be heaven!!!)