Like eulogy said, any of those 3, and along with Best Buy.
Right now Best Buy will give you $30, and if you have a Gamer's Club Unlocked Membership, which adds $3, which makes it $33. GS has the best, but you'd have to take advantage of several promotions.
GS base is $20, and will give you an extra $10 for next gen trades, and add it with the Power Up Rewards coupon that adds $5 to the trade, and using your Power Up PRO Membership to get 10%, it makes it $37 total.
If you do trade it in to GS, you can buy a used copy of AC for $43.99 plus tax if you use the birthday coupon, and with the PRO membership, it makes it $39.59 before tax. You can do the same with Injustice, and since the used copy is on sale, it'd make it $35.99 and $28.79 before tax respectively.
I really hope this helps you out!