Most interesting: Conwy Valley. The variety of scenery is amazing on such a short line, and while I feel we sadly will lose it at some point soon due to the repeated flooding it is worth a ride while it's still here. Hopefully it will at least be retained as a cycle path even if it is closed (wouldn't be as susceptible to damage in that form, as sections of embankment could be dropped to ground level to let the water through), as it would be as worthy of a bike ride as a train ride.
Joint award: The WCML as a whole, ideally via the West Midlands. A line that tells the story of the UK during a pleasant (ideally in 1st) trip from Euston all the way to Glasgow, containing pretty much all the types of scenery, and carrying pretty much all the types of people, you will get in the UK. From bustling London, to the rolling Chilterns, to Britain's "Ruhrgebiet", the industrial Midlands, to more rolling hills of Cheshire, to the beauty and industry of the North West (a diversion via Manchester and Chorley well advised), to the mountains of the Lakes and Dales, the borderland city of Carlisle (I still recall getting off there once and being welcomed with an announcement worded like that with a real sense of pride that the "big train" had come to visit) and on to Scotland's second city which has its own charm.
Most nostalgic: Euston, not changed all that much from the excitement of trips down to London as a kid. Apart from the "Euston scrum", I just don't share the disdain people have for this station and its "Great Hall", something missing from many other stations and much needed.
Most boring: The entire ECML, apart from a very short section north of Newcastle. Flat, drab and boring throughout. Might as well go ride around the Netherlands. Yawn.
Most hated: Despite growing up with them and them having a certain nostalgic charm, the PEP EMU. It must have taken a good deal of thought, and a heavy measure of sheer malice, to design a vehicle in which no seat whatsoever has a good view through the window. An utterly idiotic piece of design, even if they are reliable workhorses.
Second prize: any 20m Mk3-derived EMU or DMU for the same problem, albeit not quite as bad because the windows are a bit wider.