What is the best site to use for booking train reservations in France?

I have used both of these: Train Line, https://www.trainline.eu, & Voyages-SNCF, https://uk.voyages-sncf.com/en/#/.

Trainline.eu =the most user friendly site to book a train ticket

I'm a fan of loco2.com.

https://loco2.com

I would agree with those three.
The usual advice is to go to the train operator, in this case SNCF ( http://www.sncf.com/en/passengers ). But SNCF has a reputation of not being a user friendly site and being picky about foreign cards.

the other two ( https://www.trainline.eu/ and https://loco2.com/en ) are both resellers, but they both offer all tickets, have no markup and are easy-to.use sites.

AFAIK all other sites either have a mark-up and / or a restricted range of trains or tickets.

It is worth reading this beginners guide to trains in France. He recommends trainline.eu : https://www.seat61.com/France-trains.htm

I third the Trainline.eu recommendation. Was a godsend after fighting with SNCF's confusing and limited payment procedure.

I prefer loco2.com as it understands and does the discounts for both my Nat. Rail Senior Card (for UK) and my SNCF Carte Senior+ (for France), and accepts payments in either £ or euros for French trains.
But I do have to split the ticket if I'm travelling from elsewhere in France than Paris, right though to a UK destination.

I don't think SNCF is so bad. It use to be much worse.

For the trip you want, probably any of those sites should be equivalent. There are some specialty add-ons (which do not apply to you) that can only be booked through SNCF.

PS I'd chose Lyon over Aix in a heartbeat.

Do some research on the mistral (cold wind) before deciding to visit Aix-en-Provence in January. I think Aix could be affected.

I've never had a problem with vogages-sncf.com

for France, I use SCNF

I highly recommend the trainline app. Used it for all our train travel in Italy this past summer and will use again for our trip to France this June. You get real time updates and no service charges.

I agree with Trainline and SNCF for Tickets in France.I have had good experiences with both.
Mike

I've always used S.N.C.F , I've personally maybe only used it 5-6 times , but I didn't think it was difficult.

I understand that some ticket resellers do not show all the trains (various departures ) .

I've used Rail Europe, http://www.raileurope-world.com and it was fine, but then again I'm no expert on the matter and now looking at this post, I may try other sites next time around.

Rail Europe is not really a good option. From reports on this forum, it often doesn't offer all of the available trains (particularly not the cheaper ones??), and I think it adds a substantial service charge.

Some people don't realize that Rail Europe is travel agency and not a train company. And they are fine as a travel agency and if you are willing to pay their fees for the services they provide. Some required more hand holding and RE is pretty good at that.

SNCF has rebranded its train service:

https://en.oui.sncf/en/
I agree that the company has steadily improved its website service. Even the dread bump to RailEurope became easy to avoid. Credit card difficulties reported on this and other websites appear confined to US cards which have resisted entering the 21st Century chip-and-pin technology.

Used SNCF to book Paris-Strasbourg abd Colmar-Zurich for December 2017 trip. NO problems using my US Chase Sapphire Reserve card. I have also used trainline.eu in the past. Both good.

I used voyages-sncf.com (which apparently is now oui.sncf). I booked a trip for MSM from Paris. It was very easy. I even used my AMEX with no problem- and AMEX is usually a problem. I just had to print my actual ticket when I got to the station. Which was also easy. Make sure you have a confirmation number from the email they will send you and print at a kiosk.

I also found a cheaper rate than the U.S. based site they suggest you use...Keep the country choice at E.U. otherwise they'll send you to the RailEurope site.