My central device is a Polar FT7 with a BT capable transmitter. I've considered switching to Garmin, but I've spent too much on Polar and only Polar can do swimming heart rate well.
I manually log the gym workouts in Runkeeper. For GPS activities, in addition to tracking on the watch, I connect the transmitter to my phone and get the GPS/heart rate data tracked with both Runkeeper and Endomondo (for redundancy). Runkeeper tends to be lower than Endomondo and my watch on the calorie burn, so I modify my workouts on there to reflect the watch for consistency.
Both Runkeeper and Endomondo are synced to Myfitnesspal and EarndIt. I'm also syncing Runkeeper with Withings where I enter my weight and body fat % daily based on my Tanita scale. Finally, I sync Runkeeper to a shoe tracker to get the mileage on my running shoes.
I have Google Fit activated, but I never pay attention to it. It just pops up whenever I have a mid-long run.
That's a lot of software.