If you want a stock Android experience, CyanogenMod 12/BlissPop are excellent choices.
If you want Sense, then I very highly recommend InsertCoin. I run IC 7.0.4 on an M7, which is based on Sense 7 and it's excellent.
Remember, make sure your phone is rooted, a custom recovery is installed and that you have backed up all data at least three times along with a NANDroid backup, along with an app backup made via Titanium Backup.