I've gone and writen down your stat totals for you: Hp. 352, Atk. 471, Def. 373, Sp.A 416, Sp.D 511, Spe. 435. As you can see, your team is great at Sp.D and bad at Hp and Def. So you have a choose, you can either choose one of your worst stats, or you can continue to do great in Sp.D. If you choose Hp, then you should go for Wobbuffet. Why Wobbuffet you ask, well Wynaut =), but seriously go for Wobuffet because it may less Hp then chansey and blissey, but has a lot better defense, and without a good defense, what good is a lot of Hp. If you choose Defense, then go for Steelix because it has the 2nd best defense there is(1st is shuckle) and is a great steel type to have. If you choose to stay with Sp.D, then go for Probopass because not only does it have great Sp.D, it also has a little worst Defense, which you need. This are all fine Pokemon, but the chose is up to you.
Have fun ;)