Paris to Lauterbrunnen best via Geneva, Zurich or Bern?

The fastest train connections are usually via Basel, 5.5-6 hours total travel time, as suggested by DB train schedules.

Best = Fastest and cheapest. There is no point in taking a deliberately extra long route. Just go to www.sbb.ch enter Paris to Lauterbrunnen, date and time, and it will find you the fastest route (which is usually via Basel, sometimes via Geneva, depending on the time of day, and never via Zürich, that is a long detour). Let the computer do the route ant time calculations, not you.
Anywhere, the scenery starts after Bern, and all routes converge on Bern.
Yes, you can fly to Geneva or Zürich (nobody flies to Bern), but that will take longer from Paris city centre than a TGV high speed train would.

I agree with the others. I've usually found travel via Basel to be the quickest and easiest. The trip from Basel to Interlaken Ost is an easy two hours (especially if using a direct train), and the trip from there to Lauterbrunnen the usual ~20 minutes.

Yes. We've done that trip a couple of times. Most direct route would be Paris=>Basel on the TGV, connecting to Basel=>Interlaken (which goes through Bern).

Via Zurich wouldn't really make much sense if your ultimate goal is to get to Lauterbrunnen. Via Geneva would seem to add a couple of hours. Perhaps that route would be a bit more scenic, but personally, I'd rather spend that extra time at my destination. FYI, whichever route you choose, you'll still need to catch that final train leg to Lauterbrunnen from Interlaken Ost station.

I've also taken the train from Paris to Basel, then rented a car and drove to Lauterbrunnen. Made more sense in that particular case, as we had 4 people, and the car was a cheaper solution for that rail leg compared to 4 train tix. Added bonus was that it gave us a chance to efficiently explore Lauterbrunnen and get our bearings for the next few days before returning the car that afternoon (it was raining pretty hard for portions of that day, so we did stuff we probably wouldn't have done if we only had taken the train, like stop and walk around betweens rain showers, stopping at a roadside farm to buy some cheese direct from the cellar of a roadside farm and wander around a bit in the valley).

All depends on what you want to do... : )