Not an expert MNK, so I'm not entirely sure, but 5/5 Cobra Harness set would be a good bet probably. There's Counter+ gear, like Avenger's Earring, too.
That being said, counters CAN miss. It won't say that they miss, but it will just not proc. Capping your acc takes priority over counter+ gear. Keep that in mind.
All I know is that counter is based on accuracy, so the first thing you need to do is cap your hit rate. So it's probably only worth-while equipping in +counter gear if you're not sacrificing your 95% hit-rate to do so.