It has to be Fear Of The Dark, surely? It strikes me as the most obvious example.
I mean, for starters, how many casual Maiden and metal fans in general actually know the studio version compard to the infamous 1992/93 live version?
And I don't blame them. The live version is full of life, vigeor, excitement and is well performed. It actually makes the song itself come to life and gives a powerful and believable edge to the song's lyrics and subject matter.
The studio version, in comparison, is utterly flat and lifeless and suffers from that Barn Yard sound and production that Maiden had throughout the '90's ('though I still enjoy 90's Maiden very much, I hasten to add). Meh.