A very important question to ask yourself before choosing a fitness tracker is the type of form factor you want.
The best wearable fitness tracker is usually bracelets, watches, or clip-ons. Most clip-on devices these days can also be worn on the wrist, but not vice versa. Bracelets and watches are hard to lose. Clip-ons can fall off or get thrown into the wash.
That said, bracelets and watches can get in the way when typing on a computer or washing dishes, for example. If you're bothered by having something on your wrist 24/7, you're probably better off with a clip-on. Additionally, wrist-worn devices are not always eye-catching accessories to your outfit.
Clip-on devices are smaller and more discreet when worn on a waistband, like the Lumo Run or the front of a bra. These clip-ons don't have a display, meaning you have to rely on a smartphone to see your tracked activity.