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I'm afraid your question is opinion based and no one can really give you an answer your looking for but you.
Some things to consider are newer versions of Android have cool new stuff. Unfortunately cool new stuff runs in the background and wakes the device more often. So an older version of Android may have less stuff running in the background. But if you have root access you can freeze, or remove resource hogs. Some devs make vanilla roms. But then you may be missing the feature you want. How many apps or tasks running depends on you. You can install wake detector and monitor what's waking your device and killing your battery. In my experience Google now, Google search, and apps like Facebook just suck power so I remove them. Also using dark or black themes can greatly help improve battery life. The screen on my phone with a solid white picture can be on for 4 hours. Where it can stay on for 8 hours displaying a black picture. That's a HUGE difference! Probably because our screen is huge. Also cell signal matters. A slow weak signal drains power faster. Being on Wi-Fi helps.
There's just Soooo many factors to consider that are user based. I'm sorry I can't say "this rom is the best", but hopefully I gave you some tips how to choose what works best for you.