A heart rate monitor is best for steady state cardio. It won't help you much in terms of calories burned (if that's what you're going for) if you're doing something other than steady state cardio.
I have a heart rate monitor for use when running. I'm still not much of a runner so I run/walk, so the heart rate monitor is even not that useful for me in terms of calories burned. I use it mostly to watch my heart rate when running and keeping it in a certain range. I have a tendency to start running too fast and then I'm burned out of energy for the latter half of my workout.
Hopefully someone else can jump in regarding other types of trackers. I think it depends on your goals. For me, I only track the minutes and activity performed and am not worried about calories burned so I simply enter activity into an online application.
It might help if you could give some info on what you're interested in? Is it steps/day, total day activity monitoring including sleep, calories burned, something else? I personally don't have much experience with fitness trackers but perhaps extra info will help someone else jump in who knows more.