I always thoroughly research the games before buying so I don't really have any regrets about any of them. For example I'd have probably regretted buying Watch Dogs for full price; but I didn't. Same thing for Destiny as well. Saves you money and increases the enjoyment.
If I really push it though, I can say Resistance 2. I didn't actually pay money for it but give away Oblivion in exchange, that was a really bad decision.