I can't decide on just one! I must have several
One that I've done a couple of times (and it's not
just rail, but bear with me) is Hemel Hempstead to Bastia (Corsica). Leaving Hemel and heading into London, we then took the Eurostar (from Waterloo) to Paris, before racing across from Gare du Nord to Gare de Lyon for our connection to Marseille. Top deck first class (very cheap!) on a TGV Duplex gave a fantastic view of the changing scenery as we shot through France - 3 hours Paris-Marseille is pretty awesome! Arriving at Marseille St. Charles we got off and into the Mediterranean air - and this is something I love on TGVs down to the south. Having left a slightly muggy Paris, to board an air-conditioned train and, the next time you go outside,
feel that you're on the Mediterranean coast is fantastic. We then proceeded to cross Marseille using their metro (very...erm...plastic) and boarded the beautiful SNCM
Danielle Casanova (or, on other occasions, the equally splendid
Napoleon Bonaparte)to sail across the Med from Marseille - Bastia overnight. As close to an epic journey as I've ever got!
I've done similar things from London - Antibes - the coast line from Marseille to Antibes has spectacular scenery, but of course you miss out on the boat crossing.
Slightly shorter, but still in France, Paris Gare de Lyon - Annecy is a brilliant journey too - being aboard an 18 coach TGV (in places you can see both ends of the train when you look out of the window) snaking through the Alps is pretty special. * On an even briefer note, and one I did at Christmas, Paris St. Lazare - Rouen Rive Droite is a great journey - for railways rather than the scenery here. We were hauled by one of those fantastic reverse-rake locos (a BB15000, I think?) in one direction and (I think) a BB16000 on the return, both times in Corail comfort. The line is very busy with freight, as it follows the Seine to Le Havre, the main port for Paris (along with Rouen, I think) and I saw several 66s!
France has much more variety than the UK locomotive-wise, which makes it generally a more interesting place to travel by train, I think.
In Britain, I enjoy travelling down to Plymouth on FGW - esp. when it goes via Bath & Bristol! But No. 1 has to be my trip on WSMR late last year - Marylebone to Shrewsbury. On the way we were right behind the '67' and could see it quite easily through the carriage-end-gangway-door (whatever it's called

). The food in 1st class was fantastic too - best breakfast I've had on a train! The return was best of all though as we were following a late running special with 34067 Tangmere. At Bicester we were put onto the wrong line, where we operated for about 20 miles, I'd guess - most of which I spent at the window looking at the signals on the 'right' line as we tried to catch it up to overtake. As it happened, we didn't, but it was great fun!
* As an aside it was interesting to note that at Chambéry the TGV split in the station - nothing unusual in that, you might think, but actually it split with passengers on board
and all the train doors open - with no warning! You can hardly see such things happening in the UK even with the most basic of trains - and this is France's flagship train, the like of which (on domestic services) we have nothing to compare with!