Oh, it'd be tough. I have subscribed to dozens of them but there are three I stick with - Quilter's Newsletter, Quilting Arts, and Machine Quilting Unlimited.
I think if I absolutely HAD to pick just one I think I'd go for Quilting Arts. It's not about patchwork quilting, it's about art quilts, which I don't even really make. But I find it to be really inspiring and beautiful to look at, it's a magazine I can re-read a year later and find something to get excited about. Every issue feels very "rich". I like their sister publication, "Cloth Paper Scissors" too. I don't subscribe to that one anymore but it's just as good, just oriented more towards paper crafts.