A glass dildo with a not-so-subtle G-spot curve works best for me.
I lose a little bit of muscle control when I play, and thrusting with a glass dildo while that's happening can lead to cervical pokings - ouch! Rocking instead of thrusting works much better, so that curve is a must.
I prefer it to be smooth or only very lightly textured, since textures can be a little bit too much for me to handle - especially in glass! It also should be about an inch in diameter, maybe just a wee bit over. I don't like the idea of having any rock-hard materials be too big.
... Bent Graduate forever!