I've never smoked, but I was an alcoholic. From the time I decided that I really needed to quit, and really wanted to quit, it took me about five years before I had my last drink. (About 25 years later, I discovered that I am no longer an alcoholic --AA is wrong-- and if I have half a glass of beer, for the taste, I have no urge to drink more.)
For several years I smoked a lot of pot. Pot is not physically addictive, but it was hard for me to quit once I decided that I didn't want to go through life feeling muddle-brained.
But the really hard one was oxycodone: After taking it for (I think) about three months, the physical withdrawal was extremely harsh. I was highly motivated to get off it as soon as I could, but I was able only to cut back by about one tablet per day each week. I.e. 5 per day for a week, then 4 per day the following week, etc. And even that was very difficult and required intense motivation. But while I was taking it, it was very effective for the pain, which had been excruciating, and it made me feel really good when I was taking enough to fully counteract the pain.
I once knew a guy who had kicked a heroin habit, but said he could not quit smoking. That, he said, was just too hard.