If we restrict ourselves to tickets which are intentionally priced cheaply, rather than 'anomalies', then I think certain Advance tickets definitely come high up. Scotrail has some extremely good value Advances on their rural lines, e.g. Wick to Inverness for £5.00 (without a Railcard) is possible if you book far enough in advance. It's not a massive distance as the crow flies compared to some other routes, but it's a very scenic journey and you get 4.5 hours of travel for your money! Obviously the taxpayer (i.e. you) is subsiding this kind of journey massively.
I think Chiltern's Advances also offer very good value for money. You can do Kidderminster to London (or vice versa) from just £5.50 without a Railcard. Considering you'll be in modern refurbished Mark 3 stock, with lots of legroom, tables and so on, I think that's a bargain. Availability may be slightly better for Birmingham to London as the loco hauled services operate during the quieter (and hence cheaper) part of the day more on that specific part of the stretch.
Northern Advances can also give very good value, especially if you bag the lowest tier. But you will invariably be on older stock, which, even if refurbished, is still not particularly comfortable for longer journeys.
Actually I would argue some Virgin Trains Advances are actually the best value. Crewe to Glasgow or Edinburgh can regularly be found at the lowest tier for just £6.00 (if you book the really quiet trains). Considering the fact you're getting 2.5/3 hours on a 125mph tilting train that gets you there far faster than any other reasonable method of transport could, I think that's incredible value.
For walk-up tickets it's slightly different, but generally the ones which will be best value will either be operator-specific ones (e.g. Hull Trains/Grand Central/West Midlands Trains only) or route-specific ones (e.g. via Appleby/Uttoxeter/Llandovery) which mean you're limited to a less frequent, slower and/or less comfortable service.
