Worst game I ever bought because of friends was probably Game of Thrones: Genesis. I warned them that movie-based games are always terrible, especially strategy games. Ended up getting it on release due to constant pestering, and it was even worse than I could have possibly imagined. Close second and third and fourth are Command and Conquer 4, Command and Conquer 3, and Command and Conquer Generals. Also, Red Alert 3. Rabid fan of C&C, Red Alert, Tiberian Sun, and RA2, but the second EA bought out Westwood the series went into a fatal tailspin into complete suck. They even *ruined* Earth and Beyond, which was easily one of the best MMOs I ever played... of course, my friends and I always hoped the next one would be better so we just kept running on the brand loyalty treadmill.
Games I was surprised by, to top the list, would have to be Minecraft, Heroes of Annihilated Empires, and Valkyria Chronicles. Buddy in college pestered me for months to play Minecraft, had no interest because it had worse graphics than Half-Life. Played it once, bought it that night, and eventually ended up at Dwarf Fortress which has ruined all other games for me.
HoAA I picked up for $3 on a steam sale, played it, got addicted, got all my friends playing it, then was emotionally devastated to hear the sequels had been canned. Would have been absolutely amazing. Valkyria Chronicles I picked up used from a gamestop bin, found out it was the best squad-based tactical RPG I'd ever played and spent months hounding my friends to get it, who eventually relented and loved it. So, a bit of inversion on that one.