Best time to enter Segrada Familia and which tower ?

I went on the 19th of Sept. and at that time, there was no entrance to any of the towers because of Covid.
Make sure you are checking the official website and not one of the other ticket sellers. My guess is that the towers are still closed.
My English language tour was at 09:00, when they first opened. The sun was streaming through the windows and I got glorious photos. By the time the tour ended at 10:00, it was starting to fill up. I spent another couple of hours wandering around, visiting the museum, and taking more photos. By the time I left, it was really crowded, but Covid style. I was very happy that I was there in the morning and not in the afternoon. My photos in the morning were better, it was easier to get photos without people in them.
Ricks's suggestions are not always the best, unless someone else wants to back him up.

Tim, unfortunately I can only comment on our pre COVID visit 5 years ago. That said what I can say is we were there mid morning. It was not crowded then. We chose the Navity facade. We took the small elevator up. Although the views are not particularly remarkable we felt it was well worth the experience. We enjoyed descending the unique stairwell. If anyone has mobility issues or fear of heights this may not be an option. We did enjoy the architectural details from the view & then followed up by walking the perimeter outdoors. We did a self tour and were there at. least a couple hours. We visited La Sagrada following a walking tour with Runner Bean Tours. We took the Gaudi Tour with them. It was a great introduction to Gaudi. Highly recommend them. On another day we visited Parc Guell. We enjoyed the park very much.
I hope La Sagrada is open when you visit.
Enjoy Barcelona! https://community.ricksteves.com/travel-forum/spain/is-a-climb-up-the-sagrada-tower-worth-it

I had a similar experience pre-COVID (in 2017): Having a ticket for the first time slot in the morning got me into the church when there were no holdover visitors. It was blessedly uncrowded for a while. I'm not much of a photographer, but for those who want to take pictures, that first entry period is what I would recommend. I didn't ascend a tower, so I don't know anything about that experience other than that folks used to report here that your tower entry time was shortly after the time shown on your ticket, so those tower tickets kept you from taking full advantage of the relative emptiness of the church at the beginning of the day.

I also liked the museum, which had very, very few visitors even when the church got busier.

A possibly-important note since it sounds as if you may be traveling with a family group: Once you enter the shop, you cannot go back into the church or the museum area unless you buy another entry ticket--almost certainly impractical due to length of the ticket line. Be sure all members of your party know that so they don't wander into the shop, planning to rejoin the rest of the group inside the church later on.

I visited with a private guide 3 weeks ago on a Friday at 3 pm (sunset at ~530 pm). The stained glass windows were glorious in the afternoon sun. Can’t comment on the tower(s). Plenty of tourists but not wall to wall by any means. A transcendent experience. Enjoy your visit.