Stratford was the answer I came up with and the answer given. As O L Leigh said the Shenfield electrification was orignally 1500V DC, converted to 6.25kV AC around ten years later, and the remaining 6.25kV sections (originally put in because of worries about clearances and flashovers umder bridges with higher voltages) were all converted to 25kV by the early 1990s. The Central line which arrived around the same time uses LU four-rail electricfication, followed by the 1986 third-rail electrification of the North Woolwich branch, and the 1987 DLR bottom contact third-rail.
As it was a fairly general quiz I thought the answer couldn't be that obscure, but I was wondering if there was somewhere on the continent that could have had as many systems. If there was I would probably go for somewhere in Switzerland or for a border changeover station such as Aachen.