It really depends on the person I'm pegging. Body size and height play a huge part. Most important is that I love to be able to look into my partners eyes while he's having a prostate orgasm. Watching a man go post verbal, out of body, eyes to the back of the head is *chef's kiss*.
Generally, for slow, sensual soft pegging, I like to have my partner lay on his back or lean onto some pillows, and I have my legs under his, sort of like a scissor position. It's gentle, intimate, not a ton of thrusting or pounding, just slow, soft grinding. I find this is a good position to hit the prostate perfectly with an angled dick.
If I want to turn someone into my Fuck Toy its him on the very very edge of the bed, laying on his back, usually stacked on multiple pillows, legs propped on me at a 90 degree angle, I hold onto his legs to fuck him harder and deeper.
If I want to use my double sided dildo he rides cowgirl