I have been travelling to western Switzerland from Paris in various ways for 60 years, so I think I can give you the advice you ask for.
The ‘most scenic’ way from Paris to Zermatt would be the old traditional railway routes, but as these take more than 1 day, these will be almost certainly not be helpful to you on this occasion.
Therefore the ‘fastest’ way by TGV high speed trains will in this instance be the only practical option, therefore I will tell you which route will give you the most ‘scenic’ views. All France – Switzerland services are operated by TGV Lyria joint venture. The first thing to say is that, when a TGV is running at full speed on the dedicated high speed line, the ‘scenery’ goes past the window rather too fast for that journey to be called ‘scenic’. However on the TGV routes to Switzerland, the TGVs on all three possible routes do some parts of the journey on slower traditional tracks, with scenic views.
Therefore I will list the facets of each route for you here:
There are three practical routes Paris – Zermatt (all finish with the ride from Visp to Zermatt on the MGB narrow gauge railway:
Times: Paris – Geneva takes just over 3 hours, Geneva – Lausanne takes about 30 – 40 minutes, Lausanne Visp takes about 1 hour 40 , Visp – Zermatt takes just over 1 hour
1) Paris – Macon – Geneva TGV; Geneva – Visp direct CFF (SBB) IR train
High speed track Paris – Macon, traditional slower track Macon – Bourg en Bresse across the Bresse farmland famous for its chickens; Bourg en Bresse – Bellegarde – Geneva through the Jura mountains – very scenic; Geneva – Visp scenic Lake Geneva, mountain and Rhone valley views
2) Paris – Dijon – Lausanne TGV; Lausanne – Visp on the Geneva – Visp direct IR train
High speed track Paris – Aisy (a small Burgundy village north of Dijon); ‘classic’ route Aisy – Dijon (scenic hilly route); classic route Dijon – Dole (flat land), Dole – Vallorbe (Traverse of the high Jura mountains – scenic); Vallorbe – Lausanne (descent from the Jura – spectacular distant view (weather dependant) of the whole Alpine mountain chain; Lausanne – Visp views as per (1) above
3) Paris – Dijon – Basel TGV; Basel – Visp IC train
Not recommended, a longer way round than 1 and 2, less scenic route overall