Not sure I can pick just one, so I'll go with a few that are all actually a little similar. Zelda 2, Castlevania 2, Bionic Commando, and Strider (all for the NES). All sidescrolling platformers with at least a touch of RPG, and all with a slightly more mature sensibility and a penchant for being really damn hard at times (though CV2 is more hard due to it's poor translation). They were easily the touchstone games of my childhood and tend to still inform a lot of the gaming choices I make today. I still like games that trend a little more mature, that mix action and RPG, and that can be a bit challenging.