I personally like round watches, but for a smartwatch I think the form factor of the Apple Watch works best. You can put more info in a smaller form factor with a square than a circle. Currently, my AW is the Ultra 1, but even the older 38mm AW's packed in a lot of info depending on the face.
For example, I have a Seiko Astron. It's an analog watch but has a ton of info on the dial: hours, minutes, seconds, date, plus day of the week (on one subdial) second time zone ( on another sundial) and power reserve, airplane mode, number of satellites it connects to when referencing time, daylight savings, etc. (on a third subdial) and AM/PM for the second time zone on a fourth small subdial. That watch is 44 mm and I can't imagine it being useful in a smaller size.
A lot of the larger round smartwatches can look like a dinner plate on the wrist. So, another vote for square.