If you use it on a daily basis or have pets that will use the couch for naps - good quality leather. Or expect to replace it in 3-5 years (which isn't totally a bad thing. Great way to make a room up-to-date and like new. Buy cheaper furniture.)
If you use it only occasionally and are willing to protect it from your pets - whatever material appeals to your taste.
(
Here's what we use on our fabric and fake leather chairs. The leather couch is 11 years old, gets used every day for HOURS when we watch too much TV, and the dog loves to be there. It still looks great.)