I do enjoy these threads and the chance for people to grumble about how their local place is so much more important than the rest of the world may think it is.
Realistically though, there's never going to be a fully level playing field. Some places might be large but are on the fringes of the country (Liverpool, Swansea etc). Some places might be large but lack significant business/tourist/ long-distance-student market that makes rail travel more important (Bradford, Sunderland etc). Some places may be large but are within spitting distance of somewhere even larger/more important (Bradford, Sunderland etc). And some places are smaller but happen to be on the way to bigger places so punch about their weight (Doncaster, Crewe etc).
(also, you can get into a lot of hair-splitting about whether somewhere like Salford is a whole separate place, or about the merits of a city with generous boundaries like Sheffield versus a city with tightly drawn boundaries like Nottingham)
Based on the above, I think that the place with a legitimate grievance would be Leicester. It's not a stuck on coast (with a small hinterland), it's not in the shadow of somewhere larger on a main line (e.g. Gloucester is close enough to Cheltenham), it does have a reasonable long-distance student market (though less significant for finance/tourists). Leicester has even lost its direct hourly service to a nearby city in recent years (Coventry), so I think that the people of Leicester are the ones worthy of any gripes on this thread (though, predictably, other cities seem to shout louder!).
Is it even always desirable to improve the connectivity of everywhere to everywhere else? A box-ticking exercise based upon an obsession with giving places as many places as possible will just end up with a mess like the Northern Rail network. I'm not bothered about Sheffield getting a new hourly service to Bradford (but I would be bothered about getting a better service to Leeds, where I can connect to Bradford). Same with Cardiff (which would benefit a lot more from improved services to nearby cities like Swansea/ Bristol) than whether it has a direct train to Leicester or Newcastle. Simply ticking a box for ever one of the other cities on a list of twelve that your conurbation has an hourly service to is a pretty arbitrary way of assessing things - I think that links within a commutable distance (say an hour's travel) is significantly more important.