Porygon2 in recent months has become by far and away one of the most used pokes by top players in Adv OU, what with being a a very good substitute for a Bulky Water that, unlike them, can check Gyarados and additionally fills its most important (and unique) role in the tier by revenging Dugtrio (Trace gives it other goodies too like stealing Natural Cure from Bliss to cure a crippling Toxic), whilst additionally being a rather potent status spreader. The pokemon is undeniably OU, by both usage and (I'm very sure) common consensus, so I think the on site tier list should reflect that.
Just throwing it out there.
Just throwing it out there.