I think this one's been asked before - a search should find you some more information. IIRC the best alternatives for the city came down to Elstree, Stapleford and Southend. Denham and WW are good for west London, but a bit more hassle for the City.
My own preference would be for Elstree, a 10 minute cab ride to Elstree and Borehamwood station, then you can get a train direct to City Thameslink or Blackfriars.