Also to answer the thread topic, I usually know the games I want nowadays, so I haven't felt this in a while. I do have plenty of games where I think I'll enjoy but end up loving, or think I'll love and end up not caring much for, but usually I have a good idea of what I'll like.
Thinking back though, I'd say Fire Emblem on GBA and Tales of Symphonia are good examples of games that I got on a whim and ended up as some of my favorites ever, both currently in my top 10. Even further back though, Legend of Zelda: Ocarina of Time is basically the game that turned me into a gaming hobbyist, so that one kind of wins by default.
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