I agree with whitelunchbag, there are a lot of very good custom Sense 6 ROMs to choose from, as well as a decent collection of AOSP and Google Edition ROMs. ViperROM is definitely my favorite Sense-based custom ROM ever since the M7, and it didn't fail to amaze me on the M8. It has so many customization options and features you won't find on stock ROM, I'd say give it a shot. Before you go ahead and try doing anything, though, make sure you perform a full Nandroid Backup (back the ROM, recovery, etc.) using TWRP recovery or such.
Go to XDA-Developers and read the guides there, they'll really help you get a good head start because it's not easy.