As a kid, definitely Gerrard. We've had better players, but there can't be many who just single-handedly won as many games as he did. In more recent years, Alisson and VVD. I remember the first time watching VVD play for us and instantly realising I was seeing a level of quality I was unfamiliar with. Similarly, I remember in one of Alisson's first games he saved a free-kick that was taken in a promising position, and some voice in my head reminded me, "Oh yeah, keepers are supposed to save shots, aren't they?"
Otherwise....Klopp?