I think a lot of traditionalists would say solids, but I think it can be spiced up with a medium or small checked shirt. I kind of feel that nii, or spoo would do something interesting with it, playing with colors and mixing paterns. I think it has to look pretter deliberate to make it work, like an equally dandy pocket square, otherwise it will just look like you didn't know how to match it.
I have heard the opposite, but I feel that mixing patterns looks best when the width of the overall patterns are different. Like the wide panes with smaller checks. Whenever I see checks that are equal to the width of pinstripes to me it looks like an optical illusion.