I would consider them to both be Madonna's most iconic song. I feel like whenever I mention her to someone that's not a fan, they think of either one. Where I live, every time there's a Madonna song on the radio, it's Like A Prayer. And, whenever someone wants to do Madonna for karaoke, they pick Like A Virgin. They're both very well known, successful, controversial, signature songs. I also feel like, in the eyes of the general public, 80's Madonna is the most iconic Madonna (ageism is a factor in that choice, but that's a debate for another time).
It's a tie for me.