Stansted is madness. A train which stops when the terminal is still full of passengers! The other issue with Stn express is the physical limitation of the track layout, i.e. its only double tracked and any problem out of Liverpool st causes mayhem. LGW benefits from the Brighton mainline whch has many more paths and is not solely dependant on the overhead wires as at Stansted.
LHR is the daftest of the lot. I live 15 minutes from terminal 1 by car, and it takes 2 hours by Public transport! Unless you live in Paddington, forget it!