What is the best mod for minecraft for long term play?

I want a mod (any version) that will have maybe a levelling system or just something that i can put hundreds of hours into without completing the mod?

Download a modpack, aka a collection of mods.

Use the mod packs on Technic Launcher. I know there's at least 7 really good ones that add so much to the game. Picking classes, special skills, levelling skills, etc.