Bearing in mind that the Midland runs as far north as Carlisle via Leicester, Sheffield, Wakefield Kirkgate, Leeds, Skipton, and Appleby, I would not really class that as a short mainline.
One of my favourites is the joint Great Western Railway/Great Central Railway Alternative Route via High Wycombe, as after emerging from the tunnels in North London, you pass through Middlesex and into open countryside.
In fact, as the Stourbridge Railway was my local line when I used to reside in the West Midlands, this line became part of the Great Western Railway, therefore as I've travelled along all of the main lines from London Paddington, I would go for all of them.
This includes the first mainline London - Bristol TM, Carmarthen/Milford Haven/Pembroke, Penzance, Banbury via Reading, and the route that ran to Birkenhead via Birmingham Snow Hill, Shrewsbury, Wrexham, and Chester. I have also covered the Midland Metro trams in 1999/2000 which uses most of the original GWR formation between Birmingham and Wolverhampton.
Not so much my least favourite, but a slight cooling off is the Caledonian Railway and Lancaster & Carlisle Railway heading to and from Glasgow. This can be attributed to that I only require 5 Class 390s now and once I have cleared the class, I may consider an attempt at my 5 remaining TP Class 350s.