To add to the ECML comments, I tend to find the bit south of Peterborough to be rather more tedious than north thereof. Immediately north of Peterborough, there are several areas of rolling country on the west side of the railway which are actually quite nice to look at.
Never much been a fan of the GEML between Stratford and the M25.
Wimbledon to Leatherhead is a bit dull.
The view from Tramlink through Beddington Lane is dismal. It's a shame, because Morden Hall Park is largely invisible from the trams, which is actually pretty pleasant.
West Croydon - Sutton, West Norwood - Crystal Palace Tunnel and Redhill - Reigate are all particularly boring bits of suburban railway on the South Central area. Historically all 3 had some interesting quirks, but now they're almost entirely uninspiring. Anything through Streatham is also enough to make anybody weep with boredom.
I tend to find Shalford to Reading quite tedious as well. Also featuring lots of stations which have the potential for a long cold wait with nothing of any entertainment value in the vicinity.