Merseyrail Wirral Line around the Loop and to Birkenhead has to be a contender.
Currently there are 14 tph off-peak with an extra couple of trains per hour squeezed in at peak period.
Sure, the Wirral line diverges into 3 or 4 branches on the Wirral (as do many of the LU lines and Thameslink), but if you are standing on one platform at any of Hamilton Square, James St, Moorfields, Lime St Deep Level or Liverpool Central, you will see a train departing every 4 minutes or so any time throughout the day.
Under the Mersey is a more "homogeneous" high-frequency service than dashing about the platforms at Manchester Piccadilly or Clapham Junction looking for the next departure to Stockport or Waterloo. The core of the Wirral line is also more frequent than the Low Level routes through Glasgow Central or Queen Street.