London-Edinburgh (West coast vs east coast. West Coast is slower but cheaper)

Bristol-Manchester (XC via Birmingham vs ATW via Hereford - I usually take the slower cheaper Wales train as I prefer the route and the seating)

Manchester-Norwich (via Sheffield-Peterborough, Via London, Via Leeds-Doncaster)

London-Reading (GWML from Paddington vs South West from Waterloo)

Exeter-London (Paddington via Westbury on GWR vs Waterloo via Salisbury on SWT)

Manchester-Warrington (ATW via Newton, EMT via Birchwood)

London-Hastings (via Tunbridge Wells or via Haywards Heath)

And different competitors on the same track

Milton Keynes - London (West Coast, London Midland, Southern)

York - Newcastle (TPE, LNER, XC)

Manchester-Glasgow (Northern+WestCoast, TPE)

Stoke-Manchester (Northern vs XC vs West Coast)