A great waller is Umbreon. The usual Curse-Moonlight-Payback Umbreon is great, and few moves that are used in competitive battling often that are special are super effective. (I can only think of Signal beam, Bug buzz, Aura Sphere and Focus blast). Another Wall is Vaporeon, with Aqua ring. Since you have diamond, Pokemon cannot have hidden abilities, so Magic Bounce Espeon wont work, but even with no Magic Bounce, if you give it defencive EV's and teach it moves like Reflect and Light Screen, it can be a half-decent wall. At the end, I reccomend Umbreon, with amazing Defencive stats and decent attack.
I hope this helps! -Jellohamster