Best firmware for 935

Wanna know too kk

My best firmware was 12.8x but more important, gps version pre 2.50. From gps 2.50 and up I experience a 10% battery use per hour (previous more like 5% per hour).

There are options to downgrade the firmware, but unfortunately it is not possible to downgrade the gps version. 

To be honest, I wouldn't bother with downgrading the watch firmware. Not much difference between the versions imho.

More specific, heart rate incorrect? Hmm. haven't heard other people complain. Might as well be hard ware related. Do you have cracks in the heart rate monitor (like so many of us)?

I can't say what version is the best. But I can tell that 14.10 is fcked up. Elevation is broken. I compared the same ride with my wife's Fenix 6X, and F6X is ok, mine is not real. I don't want to update to see more and more bugs. Garmin is not serious company.

No, I have no damage, no scratches on the heart rate monitor

Well, if heart rate is important for you, use a cheststrap. Optical heart rate doesn’t work very well (on any brand of watches)

Last friday eve I had a 10k track race. I run at my max, so heart rate monitoring isn’t important and I didn’t want to race with my cheststrap.

Afterwards I looked at my heart rate, recorded with the optical reader of my watch. Unit 5k it looked fine. Just above 160 bpm, which is good for my age. Then it dropped to 130ish. Which is complete nonsense. 
My watch was snug enough around my wrist. Maybe the cold weather had something to do with it.

I have used OHR of different brands for a number of years. None of the. Is reliable for me. 

I've been using 15.10 with no issues and haven't had a need to upgrade and won't unless there's a new compelling feature offered (once bitten twice shy).  I use the watch for running outdoors, running indoors, trail running, cycling outdoors, cycling indoors, downhill skiing, and weight training workouts.  15.10 has been working perfectly with my array of sensors, etc.