I'd have to say color because it's the first thing that draws the eye, then design because if the color and design fit together that's a real pull. Much as I hate to say it, you can't really see the workmanship til you get close, but after I've said that, it's the workmanship that makes me think that what was a very beautiful, stand out quilt from a distance is in fact "a work of art".
Don't forget that over the years, color fades, patterns become over produced and a ton of people are doing their version of same patterns (almost all of which are 'Quick & Easy').
Workmanship will always be there, the number of pieces, the handwork, the quilting stitches and how even they are, the originality of the design, the overall quality of the quilt. Even after hundreds of years go by and there are only parts of the quilt left, you can look at something and say what wonderful workmanship.
All that said, someone posted a picture of an old quilt with a VERY original version of a design and it just grabbed me (I think she was asking if anyone knew what the pattern was called). The colors were ok, I couldn't tell how good the workmanship was but the design reached out and screamed 'LOOK AT ME' to a person who loves quilts.