Feels amazing if you're into it. The best dog is any giant breed that loves you and wants to have sex with you. They normally react positively and by asking for more sex if you do it gently and the way they like it. Look for the guides on female dogs for more information.
I've heard they can become accustomed to a male partner to the point they will eventualy come to you for sex because they start to enjoy it for pleasure not reproduction.
The best dog to do it with is any that love and trusts you, and that is big enough for you to fit. If you're going to get a dog, get the biggest one you can because there is no guarantee you'll fit in any dog. They're all different. Only rule is the bigger the dog the better your chances.
For the rest, I suggest you use the forum search feature. Way more info has already been posted than people will post here.
