Best time to visit Paris?

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I am not sure if there is a less crowded time anymore, especially if somewhat pleasant weather is desired. That goes for most major destinations in Europe. Sure, November through the first part of March might see fewer tourists, and August used to be a bit dead, but be prepared to book tickets ahead anytime.

Best time to visit Paris is Spring and Fall.

For Spring - I would suggest April, May, and June
For Fall - Start second week of October to the end of the October and November

We were there in mid April this year and the weather was practically perfect. Temps were great to a little cool and everything was blooming. We had a couple of random rain showers, but were able to duck in a cafe for a few minutes and then it cleared.
Having never been to Paris before I have no benchmark for crowds, but what I would presume are some of the typical tourist spots - the Louvre, Versailles, Notre Dame, etc. were very busy; however we were able to get tickets to the Louvre and Versailles (before we left the states) and see everything we wanted to see therein. It was the week of Easter so Notre Dame had altered hours and we decided to try and see the inside on our next trip.

Complicated answer...do you want the "best" time or the "less crowded" time? Spring and fall are the "best" times but they are certainly not the times when it's "less crowded", since they are the "best" - I've been there at various times of the year and January is quite dreary and cold and not crowded, so the weather could be a deterrent for you depending on where you are coming from and what your preferences are. Just be sure to be bundled up with a hat as it's very windy. If you go at the end of January you will be there during one of the two sale periods so you can do some good shopping! If you are coming from the US and are tracking the plummeting exchange rate (not in the dollar's favor) that might be in your favor, maybe it will keep some American tourists out of the equation and thin out the crowd a bit. I say instead of overanalyzing, go when you can and have a great time!

I went mid February and liked that time. But I'm from the Puget Sound (Seattle area), so the temps were actually warmer there then home and I didn't notice it being cold. My home is beautiful in the spring with the flowers, so I didn't feel like I missed too much not spending time in their gardens. The crowds were very manageable. We walked a lot, enjoying the outdoors (architecture) in addition to the museums. Perhaps it might have been too quiet in some areas. There is a vibe that comes with tourists.

With temps hitting 100 degrees in Paris, it may be both the best of times and the worst of times to be there this week.

As others have said, it depends... on the weather, on the crowds, on whether there are strikes going on. All you can do is pick a date that works for you and hope for the best.

I was in Paris the first week of April this year and fully expected chilly and grey weather. But we wound up having the best weather I've ever had visiting Europe (and that includes over 30 years of travel). It was perfect but I would never have anticipated that.

So it really depends on a number of factors. If you go in August, many locals will be out of town, which makes it less crowded. September is the most expensive month to visit Paris. You could shoot for May, which could have good weather, or it could rain every day. You just don't know.

The last two timesI was there were in February and March.
Not crowded, plenty to do.
Lots of different weather but with layers you’ll be fine.

Yes we were very fortunate for weather this April! It was perfect. Also consider when the time change happens. The evenings will be pretty dark until then. We arrived just after they had changed (a little later than the US does). But the streets were well-lit anyway late at night so maybe that's not a factor for you and I guess it depends on what all you want to do. I really enjoyed seeing the flowers in the gardens and the flowering trees. It was a beautiful time to be there.

The highest peak seasons are May June July December. April is good, March, September, October, November also. To me the best time is April.

Ignoring weather, the least crowded for tourist sites is last two weeks of November, first two weeks of December, January and, February.
Weather: April, May, end of Sep, October. We used to be able to add more months but with the changing climate June and early September are dicey now.

Mid-October is Fashion Week; very expensive. End of November to Christmas is the holiday markets; book early. January - Feb. is cold and wet; great for indoor stuff. March and April are prime time for trade shows. Sometimes I think the best time to visit was last weekend. (I was there last weekend, very touristy, expensive, had a good time.)

For spring, I would suggest April to mid-May (by late May/June, many children are already on holiday, and tourists start flooding the capital).
For autumn, October and November are lovely, but avoid the school holidays (Toussaint, from 18th October to 3rd November).
If you need more information, check out this article, it’s quite comprehensive: https://www.theparisianguide.com/best-time-to-visit-paris/

We were there the last week of April. Weather was fine. There certainly were plenty of people but I wouldn’t call it crowded