I would Say Porygon2 it has great special attack, and great defences with eviolite. Here is a set you should try out.
Porygon2 @ Eviolite
EV's: 252 Sp. Att/ 252 HP/ 4 Spd
Tri Attack is for STAB, Ice Beam+Thunder Bolt+ BoltBeam Combo, Recover is to get back some health. The EV's make Porygon2 have good Special Attack and still have decent bulk.