what's the best way to calibrate altimeter DEM or GPS?

Both have issues. If you're on a flat surface DEM is better. On the mountains, GPS may be better than DEM.

I would say none of them. I mostly calibrate manually when I know the altitude. If I for some reason need to calibrate using DEM or GPS and check both of them and use the one that seems most reasonable. Make sure to give the GPS enough time for both them to acquire enough satellites ang get a good fix. A few rules:

• In bad GPS situations none of them will work since both use GPS to get your current position. 
• As gspinoza said DEM may have difficulties in steep terrain.
• In open terrain both probably work.

If I plan a longer trail or hike I always check the map and try to find places where I can calibrate manually using a valid source.

As an example this is piece of a map shows a popular hiking trail in north of Sweden and as you can see there are a lot of verified altitude measurements on the map, I've just highlighted a few of them:

I’ve disabled auto calibration outside of an activity since it wasn’t doing anything to help. It helps if you know the precise elevation of your home (based on maps). Calibrate it manually at home, then keep an eye on the altimeter as you go about your daily life. I find I don’t get much drift when I return home - maybe +/- 3m which is acceptable. I’ve also stopped showering with the watch on as I’m paranoid the sensor is getting wet and messing up the barometer. Seems ok so far but I’ve only had my 6S Sapphire for a week. I’ve found with previous watches (935, 645) the accuracy of the barometer deteriorates with age, or maybe I was just unlucky and had defective units. It does seem better with the Fenix 6 line as they have topo maps built in.