Nature: Serious (this is my pref, but, I recommend getting one that has speed hindered)
EV: 252Hp/120Def/120Sp.Def/16 Sp.Atk
It's no secret that Porygon2 with Evoilite is the way to go. His defenses skyrocket, and has good Hp that benefits from EV's a lot. It's a very well rounded but horrible speed . I have solved this problem by making a Bulky Trick Room Porygon of Doom. It benefits from prolonged battles. Recover for when somebody does finally put a scratch on P2. Trick Room, although he's not the slowest, he's close enough. Charge beam + download is why you can afford to dump your EV into Hp and Defense, and slightly put some into his naturally good Sp.Atk. The final move is your choice, I usually lean towards psychic depending on who else I'm using in the Trick Room setup. With his good health and near uber defenses, getting 1-3 charge beams in during doubles is easy, and in singles, proper use of Download: porygon can survive multiple swaps, and you have recover. In case someone lands a strong crit. Focus Blast from Mega Charizard Y did a stunning 50 damage to Porygon2 the other day. There is no Ohko'ing this guy.