WMW and RI are my favourite but some readers have mentioned them already. Most of the readers who read dark novels tend to like them because a dark MC has higher points in brain department and a decent amount of maturity (god I hate whining MCs). Considering this point, I have liberally added some novels which don't have absolutely dark MCs yet have mature and calm headed MCs who can THINK ( MCs who can think are endangered species)-
1.Immortal Executioner- Great plot, eloquent MC ( i mean, he literally saved himself from trouble using wits rather than power many times.). MCs who have second chance but don't make proper use of them? Well, this one uses his second chance so well, that he literally stops using his most powerful weapon to gain more experience and regain his sense of danger that he used to have in his previous life (though he never puts himself in danger that he can't cope.).
2. Immortal- A grandpa ( MC is 70 Y/O at the start of the story.) who starts cultivating. Absolutely marvelous novel with amazing plot and quick witted MC.
3. Renegade Immortal- A masterpiece and one of my most liked novel. MC was originally nice and a normal kid however his personality turns dark and ruthless due to an incident. MC is a perfect example of 'A flawed good man'. Villains have a rare thing called rationality and actually act their age ( in most novels, villains with over a 1000 years of age acts as childish as a 10 Y/O kid with common dialogue like 'HOW DARE YOU'.)
While you are at it, please do check out my original named ' Almighty Ascension System' and support it if you like it.