I'm going to take this question in two different ways. I'm going to say my favorite body mod on myself and then my favorite body mod in general.
I think my favorite body mod on myself had to be my septum piercing. Before I got it I had the worlds longest debate with myself that lasted two years. I didn't want to spend money on something that was going to look stupid or that I would have to take out because of work. About 3 weeks before I actually got it pierced I started watching youtube videos of how people hide them. Finally, I walked in to my usual tattoo shop and asked about the piercing. It was only $30. I sat down at their chair and she put the clamp on my nose, which surprisingly I didn't feel at all. When she put the needle in I didn't feel either so it's the easiest and least painful piercing I've ever had. After a day of having it in I realized how much I liked it and instantly fell in love.
I think my favorite body mod on other people is stretched ears. I don't know what it is about it but I enjoy the look of them a lot. If my ears were so tiny that my piercer laughs at me just to get the regular old earring piercing I would probably consider getting them.