Stock Ones on the M8 work very well. It seems they got the front end working with them together it makes a great ride. I remember on my 2016 I had wanted to install them on my RGU. Dealer said not to unless I did the front also. so I passed on it.
The air worked out ok for me once I played with the air a bit.
I changed to them on my 2013 FLHTK. For me, It was a marked improvement over the air shocks. The are a good match for the stock forks. If you get them new they do need to break in. They will seem stiff at first but will get more plush with some miles.
I found a set of new take off 12" premium adjustable shocks on eBay. I put them on my 2012 Ultra. I don't really notice a difference in ride height but there is a big difference in ride comfort. I see a couple others don't think the 12" are better than the air shocks. I don't agree with that. I found them to be very noticeably better than the stock air shocks.
I bought a set of 12" premium adjustables used last summer. I think they're an improvement over the stock air shocks. I don't get the negative comments about these. I've never bottomed them out. Solo I set no preload at all. Two up with wifey 6 turns on preload. She even noticed the better ride. I weigh 200 lbs. I got them in the forum classifieds for $250.00 shipped. That seems to be around the going rate for them. I'm happy with them.
My King had the 12'' adjustable shocks and I never did get a decent ride no matter how I set them. Installed the SUPER SHOX and really like the ride now. Wife comments on how much better the ride is as well. IMO the 12"' are not good at all but just my opinion.....