The run into Liverpool Street from about Chelmsford. Particularly if looking out of the left hand windows. Mile after mile of horrid suburbs, dreary housing estates, run down maisonette blocks and dodgy garages etc. Made doubly worse by the slow trundle on the congested railway, so you get plenty of time to gawp at it all.
That used to be the intersting side from a railway point of view:
Road Machines monorail at Brentwood Sewerage Works
Speedlink wagons at the warehouse at Gidea Park (now flats)
Coal wagons at the junction of the Upminster branch (now flats)
OHLE maintanance trains at Romford
At various uses for the yard at Goodmayes over the years and the banana warehouse, usually with a solitary wagon.
Milk wagons at Ilford if you weren't distracted by the depot on the other side
Coal yard at Manor Park
Narrow guage at Stratford (and a lot more on the BR side on your right of course)
Commercial boat trafic on the Lea
Aggregates at the depot overlooking the Regents Canal
Finally a peek into the Underground at Shoreditch.
All gone now of course and a much less interesting journey.