Good question. I think the photo probably does not represent your suit completely accurrately, but I would say a solid color tie --maybe the green that Levoosh have on yesterday. Maybe a stipe like like Manton's --but then I think your square is giong to be off.
I think I may have the thing. Next time I wear this suit, I'll try for a third time and perhaps it will be charming.
Rags, going to have to call this a miss. The cardigan is throwing everything off. The pick stitching on the chest is pretty bad. And the fact that it's another pattern (granted hiding under the solid piping strip) adds all kinds of confusion. Lose the sweater and I think you have a much nicer fit. I also think a pale blue shirt would be better with greys and browns.