The chaos gods are all distinctly unpleasant to serve, but I'd say Tzeentch and Nurgle leave you the most unchanged mentally, the trade off being you need to be of the right mindset to serve them in the first place while Khorne and Slaanesh will just warp your mind into their preferred form.
They dick over everyone but actual boons are rare, so the question is who is the most likely to not dick you over horribly or is most likely to give you a useful boon.
Khorne is more or less automatically out sadly as he only views people as warriors or as slaves/victims for those warriors. Unless you're willing to beat people to death with their own babies you're unlikely to do well with Khorne, and I say that as a Khorne player on tabletop.
Slaaneshi cults can spend centuries doing nothing but getting their hedonism on, but the mind warping effects of Slaanesh worship make it questionable if you really remain 'you' for long. Nice enough if you like drugs, whips and chains, dubstep and deranged pleasures no man should know. You may die as part of a blood orgy, but you'll probably die smiling.
Tzeentch cults can likewise spend ages doing little beyond plotting and low grade violence, but unless you get put into a position of political power or become a very powerful sorcerer your actual quality of life is going to remain the same or get worse. Add on the slow build of paranoia and ambition and things rarely end well for the average cultist, especially if the bossman wants some volunteers for experiments and rituals.
Nurgle warps you the most physically, but leaves you reasonably intact mentally. His cults are relatively active, so a bad choice if you mind hurting people. You do get to ignore most ill effects of illness and injury, but it's more as a result of ignoring or embracing pain than not feeling it. Your life becomes a drudging thing of low grade unpleasantness between wars made better only by your friends and family who also serve Nurgle and the temporary relief of symptoms from things like scratching rashes or popping boils.
I'd really rather get a bolt shell to the head than actually serve one, but if I had to choose I'd pick Nurgle or Tzeentch. Both suck from the normal perspective, but at least there's a chance of a long life without major psychological changes to my mind.
If we have a common enemy? Quite a lot. And guess what? We ALWAYS have a common enemy.
Kharn kills hundreds of his own mortal cultists every time there's not an active battle, and murders entire squads of berzerkers out of bored bloodlust. Everyone is an enemy to him except possibly Angron. He only tolerates other Khornates and chaos followers because he appreciates that he needs some of them around to facilitate the Long War. It's literally part of his mechanics that he kills his own men in battle.