Welcome to the new and improved forum!
As indicated, your question has a lot of opinions from different folks, and I have mine - which of course I'll gladly share with you.
I've been into HO since the early '60s, and have used Atlas flex track and turnouts almost exclusively. My reasons are simple........ It is a quality product, readily available, and the price is relatively inexpensive (shop around on the net for significantly better prices than you might find otherwise).
I'm currently building a new HO layout, and am using Atlas products. FYI, I used them on my last layout (11x15 two level), and once correctly mounted I never had a track/turnout or switch machine failure in the entire life of the layout - 1993 thru 2008.
I've always used code 100, but if I were 20 years younger (I'm 66) with comensurate eyesight and manual dexterity, I would use code 83.
Now please understand, I am not saying Atlas is the "best" per se, for other brands are more realistic and there are even higher quality products out there. But they are more expensive, and can be difficult to obtain. So if money is not a consideration, and you do want a "better" product, then I would look at the Peco / Walthers / Micro Engineering, etc.