My spouse and I really like the adjustable, Velcro-based *Sports* Loop band. Unlike other bands, it allows you to easily adjust the band to a comfortable AND snug fit. You can also easily redo that during the day if you'd like — for a better fit, or if it's hot, or your wrist were to swell a bit. The better fit can in turn yield a better O2 or HR reading, etc.
Never have to separate the band from the Watch, either!
Also, certain bands, e.g., the *Solo* Loop, require selecting a 1-12 wrist size fitting, in advance, so it's important to get that right.
Finally, other bands with fixed holes often put you between a rock and a hard place — or, rather, exaggerating, between a vise and a hula hoop! 😎 Depending on your wrist size, you might either choke the wrist or flop the band around.