I have three main ones that I use.
Everday, I use my Fitbit. I previously had the Flex and now have the Charge HR. I love the accountability it can make me have, especially when I'm in a challenge with other Fitbit users! It really makes my competitive side come out and can push me to get more steps. I will admit I rarely get to 10k unless I have a race and work in the same day, or a race that's a 10k or longer. Sometimes I get scared when it buzzes at me! My only complaint is that the HR monitor doesn't show nearly the burn/HR it should, but I know it's not entirely accurate and use it more for overall daily burn.
When I run, I use my Garmin Forerunner 220. I had the 10 before, but wanted a fancier device. Mostly I wanted to eliminate the need for both a GPS device and HR monitor. This was at a time I was very involved with calories in vs. out and had my Fitbit Flex. I love the HR monitor because I can see if I'm pushing too hard. I also love that it vibrates for my run:walk interval, which the 10 didn't. Sometimes at races there are other people using the Galloway method and I need to know which beeps are mine!
The last device, which I don't use as much anymore, is my Polar FT4 HR Monitor watch. Like I said, I used to be very involved with calories. Sometimes if I'm going to be in an intense cardio session in the gym I'll want to know how many calories I burn, etc. This one is mostly getting put on the back burner for now but I do love it! It's super lightweight and gives good data.