I personally prefer the WCML, with its better rolling stock, better scenery and better fares, but there really isn’t any question as to the premier mainline - it’s obviously the East Coast.
Its stations are just hit after hit: Kings Cross is a major tourist attraction in its own right, and one of the most recognisable stations in the world: York, Newcastle and Edinburgh are similarly iconic and proud. The NRM has no shame in being positioned along it, and LNER are super keen on their historic associations and shiny glamour. Let’s not also forget the ECML’s dotting with ironwork milepost things - this is an ‘auraful’ mainline.
By contrast, the WCML seems almost embarrassed by itself. Euston is just depressing and needlessly poorly run, Birmingham NS seems to hide its existence, and the likes of Crewe etc. are in a dismal state of repair. Avanti takes little interest in their history, their livery and uniforms are dull and certainly not glamorous, and features like Shap Summit are completely unmarked.
For a time, though, I think this was reversed. In the late 2000s/2010s, whilst the ECML was witnessing the collapse of multiple franchises, a huge deterioration in onboard standards and catering, the destruction of one of the most stylish brands in railway history and possessing an old and increasingly scruffy fleet, the WCML was a beacon of privatised success. Innovative modern trains increasing frequencies and slashing journey times, a glamorous, trendy aviation-associated brand owned by a high profile British billionaire, high standards of onboard service - it was a mainline that was something to be proud of, not ashamed of, like the farce on the ECML. To prove the point, did anyone really mourn NXEC or East Coast? But Virgin Trains was quite publicly lauded when they (temporarily) lost the franchise in 2012…